Physics The Pioneer Science II Light Electricity [Reprint of the 1941 original in two volumes ed.] 486-60566-3

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P YS IC S THE PIONEER SCIENCE

LLOYD W. TAYLOR

Volume 11 ELECTRI CITY LIGHT

PHYSICS

SIR ISAAC NEWTON 1642-1727

PHYSICS THE PIONEER SCIENCE

By

LLOYD WILLIAM TAYLOR With the collaboration, in the chapters on modern physics, of Forrest Glenn Tucker, Oberlin College.

VOLUME II

LIGHT ELECTRICirfy

DOVER PUBLICATIONS, INC. NEW YORK, NEW YORK

Copyright © 1941 by Lloyd William Taylor All rights reserved under Pan American and International Copyright Conventions.

Published in Canada by General Publishing Company, Ltd., 30 Lesmill Road, Don Mills, Toronto, Ontario. Published in the United Kingdom by Constable and Company, Ltd., IO Orange Street, London WC2.

This Dover edition, first published in 1959, is an unabridged and unaltered republication of the first edition originally published by Houghton Mifflin Company in 1941. The original edition appeared in one volume and this Dover edition now appears in two volumes.

Standard Book Number: 486-60566-3 Library of Congress Catalog Card Number: 60-947

Manufactured in the United States of America Dover Publications, Inc. 180 Varick Street New York, N.Y. 10014

CONTENTS

Light 29. 30. 31. 32. 33. 34. 35. 36. 37. 38.

ELEMENTARY PROPERTIES OF LIGHT ILLUMINATION

.

IMAGE FORMATION . SPHERICAL REFLECTING AND REFRACTING SURFACES SIMPLE LENSES AND THEIR SHORTCOMINGS

.

PROPERTIES OF PRISMS COLOR LENGTHS OF LIGHT WAVES SPECTRA POLARIZED LIGHT

•

. . . . . . . . . .

397 410 423 435 454 471 488 504 530 549

Electricity 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.

. 577

MAGNETISM

. 599 CURRENT ELECTRICITY . 618 THE MAGNETIC EFFECT OF A CURRENT • 630 SOME ELECTRICAL UNITS AND THE MEASUREMENT OF CURRENT • 644 STATIC ELECTRICITY

OHM'S LAW AND MORE ELECTRICAL MEASUREMENTS ELECTROMAGNETIC INDUCTION DYNAMOS

•

•

. 663 • 682 . 697

ALTERNATING CURRENTS AND OSCILLATING CIRCUITS

. 713

THE TELEGRAPH AND TELEPHONE

. 730 . 744 . 761

•

RADIO COMMUNICATION THE DECLINE OF THE MECHANICAL VIEW OF NATURE

.

VI

51. 52. 53. 54.

CONTENTS CATHODE RAYS AND THE ELECTRON X-RAYS AND RADIOACTIVITY TO

1913

QUANTUM THEORY AND ATOMIC STRUCTURE NUCLEAR PHYSICS

.

. . . .

769 790 808 833

APPENDIX

List of References The Meter-Kilogram-Second System of Units Periodic Table . Mathematical Tables

XI

• XlV .

XVI

INDEX OF AUTHORS .

XXVll

INDEX OF SUBJECTS .

• XXXI

PHYSICS

LIGHT

'' The invaluable agent of our best knowledge of the environing world, and yet itself unknown except by inference; the intermediary between matter and the finest of our senses, and yet itself not material; intangible, and yet able to press, to strike blows and to recoil; impalpable, and yet the vehicle of the energies that flow to the earth from the sun light in all times has been a recognized and conspicuous feature of the physical world, a perpetual reminder that the material, the tangible, the palpable substances are not the only real ones .... "The history of optics in the nineteenth century, from Fresnel and Young to Michelson and Rayleigh, is the tale of a brilliant series of beautiful and striking demonstrations of the wave-theory, of experiments which were founded upon the wave-theory as their basis and would have failed if the basis had not been firm, of instruments which were designed and competent to make difficult and delicate measurements of all sorts -from the thickness of a sheet of molecules to the diameter of a star - and would have been useless had the theory been fallacious .... The wave-theory of light stands with Newton's inverse-square law of gravitation in respect of the many precise tests which it has undergone with triumph; I know of no other which can rival either of them in this regard.'' K. K. DARROW (71 :108)

CHAPTER29

Elementary Properties of Light ... light travels in straight lines called rays. EUCLID, 300 B.C.

The Beginnings of the Study of Light opinion of Plato, to pry into the mysteries of light was to encroach upon the prerogatives of divinity (214:238). This conviction, however, apparently did not cause him to refrain from extended comments on the subject (198:42, 70), a. variety of speculation which was freely shared by other writers. As was the case with other aspects of "philosophy," light was the subject of verbose speculative treatment at the hands of a number of the ancient philosophers. The empirical approach was not entirely IN THE

FIG. 212.

ARCHIMEDES SETTING FIRE TO THE ROMAN SHIPS

398

LIGHT

lacking, however, even among the Greeks, for in Aristophanes' Comedy of the Clouds, performed in 423 B.C., there is a clear reference to a burning glass with which one of the actors proposed to obliterate at a distance a legal document, inscribed on wax, which was working to his disadvantage (16:1 :337). The tradition, also, that Archimedes, two hundred years later, set on fire the ships of the Roman besieging force around Syracuse - supposedly by mirrors, though their design and arrangement is still a subject for speculation - is known to every schoolboy. From these meager beginnings experimental knowledge about the behavior of light has accumulated, though with certain major interruptions, through all of the ensuing interval of 2500 years. Strange as it may seem, as far as physics is concerned, scientific curiosity prior to the beginning of the scientific era was evidently confined to mechanics and light. It is scarcely surprising, therefore, to find among the principal contributors to early scientific knowledge about light such men as Archimedes, Ptolemy, Kepler, Descartes, Newton, and Huygens. They and other men who occupied the center of the stage in mechanics will continue at the focus of attention as the study of light progresses.

Rectilinear Propagation of Light It is commonly stated in textbooks that light travels in straight lines. The statement requires clarification, but is based upon a volume of common observation which extends back so far that it is impossible to identify its first author. Light is commonly contrasted with sound in this respect. It is pointed out, for example, that after passing through an aperture light continues directly on and casts a sharp shadow, whereas sound billows out in all directions and is audible, not merely in the direct line of the source and aperture, but at all points on the far side of the obstruction. There are, however, many apparent exceptions to the rectilinear propagation of light; so the thoughtful student is apt to feel that to any statement that light travels in straight lines one must add the qualification "except in the many cases in which it doesn't." It would, of course, scarcely show discrimination to point to reflection and refraction as examples of such exception, since, after all, light does travel in straight lines between its points of contact with the reflecting and refracting surfaces. A somewhat clearer example seems to be the curvature of rays from the setting sun as they pass through an atmosphere of gradually changing density. This curvature not only causes a distortion of the outline of the sun from circularity, but also makes the sun remain visible for many minutes after it has actually sunk below the horizon. The latter phenomenon occasioned a great deal of thought among ancient and medieval writers, especially among astronomers, who had to make corrections for errors in the observed positions of stars brought about by the same

ELEMENTARY PROPERTIES OF LIGHT

399

FIG. 213. DISTORTION OF THE SUN NEAR THE HORIZON (From College Physics, by Mendenhall, Eve and Keyes. D. C. Heath and Company, publishers.)

cause. (The errors were never actually zero except for stars at the zenith.) It was, in fact, Newton who made the first really accurate table of atmospheric refractions at every degree of altitude. 1 It is a matter of interest that Cleomedes in the first century A.D. was one of the first to suggest that the sun remains visible for a time after it has actually set. He so far failed to envisage one of the natural implications of his contention, however, that he rejected as incredible the accounts by his predecessors of a total eclipse of the moon during which both sun and moon were visible above the horizon simultaneously (234:2:388). A modern device which seems to cause light to travel in curved lines is the illuminator used by surgeons and engineers by which light traverses a curved transparent rod to illuminate small and inaccessible cavities. This is not, however, a phenomenon of refraction, as was the case in the curved path through the atmosphere, but is a phenomenon of what is termed total reflection, which will be discussed in Chapter 34. The same principle is sometimes used in the illumination of fountains from beneath. In such fountains light escapes from the interior of the jet only at points where the continuity of the jet is broken. But, notwithstanding these apparent exceptions, the doctrine (if it may be so termed) of rectilinear propagation of light will probably continue indefinitely as a generalization on the main trend of commonly observed phenomena. It is not really a scientific "law," but within its limitations 1

Philosophical Transactions (abridged), 6, 519 (t 721).

400

LIGHT

it is a usef.ul concept and as such finds numerous applications. Too rigorous an insistence on its universal validity can, however, lead to difficulties, as Newton realized when he tried to incorporate this doctrine into his formulation of the essential properties of light.

Speculations on Speed of Light A second ancient source of perplexity about light was the speed of its propagation. Empedocles (ca. 49D-435 B.c.), the first Greek philosopher to give serious attention to light, did not credit it with instantaneous propagation as did many later philosophers, but believed that it moved with a finite speed (227 :1 :87). Pliny (23-79 A.D.) (227 :1 :249) was more explicit. He remarked that the speed of light was greater than that of sound - a deduction which any child can make from the common observation that one can see a distant hammer fall an appreciable interval before one hears the sound of its impact. A thousand years later Al-Biruni (9731048), whom Sarton considers one of the greatest scientists of all time (227 :1 :707-8), declared that the speed of light was immense as compared with the speed of sound. And 250 years after this, Roger Bacon expressed his faith that, though the speed of light had proved too great to measure, it was nevertheless finite (25 :2:489). This would appear to be all that could be expected, in the absence of experimental data. Unfortunately the lack of such data made it impossible to decide between this prophetic opinion and the opinion, which was also widely held, that the propagation of light occurred instantaneously. Both Kepler (145 :9) and Descartes (quoted 214: 125) held this opinion, though the latter, oddly enough, also promulgated the irreconcilable doctrine that the speed of light varies in different media. The first attempt actually to measure the speed of light was made by Galileo (101 :43). He undertook to determine the time required for two observers to flash lanterns back and forth at each other, when they were stationed nearly a mile apart. He expected thus to secure a measure of the speed of light. It is unnecessary to remark that the experiment was unsuccessful. It is important, nevertheless, not merely because it is significant historically, but also because some of the later successful determinations involved the same principle - sufficiently refined to cope with the prodigious speed which light possesses.

First Measurements of Speed of Light The first measured value of the speed of light was reported to the French Academy by Olaf Roemer (1644-1710) in 16p6. Roemer had been studying the motions of Jupiter's moons and ha4 observed that a greater time elapsed between successive passages of one of the moons behind Jupiter

ELEMENTARY PROPERTIES OF LIGHT

401

during the six months when the earth was receding from Jupiter than elapsed during the other six months when the earth was approaching. Reference to Fig. 214, taken from Roemer's memoir, will help to show how this discrepancy could be made to yield a value of the speed of light. Suppose the earth to have been at L when one of the moons emerged at D from the shadow of Jupiter, indicated at B, and that the time was noted. Similarly the times of subsequent emergences were noted as the earth moved from L to K. At each successive emergence, the light from D had to travel further to reach the earth than at the previous emergence, and the apparent intervals between the emergences were correspondingly greater than when the earth was at H. By a reversal of the F same reasoning, the apparent intervals between K eclipses were less than the correct value while the earth was traveling from F to G. The difference between the mean values of the FIG. 214. THE BASIS OF interval when the earth was on opposite sides RoEMER's OBSERVATION OF THE SPEED OF LIGHT of its orbit would clearly depend on the speed (From his paper of 17 30.) with which light traveled. Roemer concluded that light required 22 minutes to travel across a diameter of the earth's orbit, thus giving a value of about 227 ,000 kilometers per second for the speed of light. In comparison with the later more accurate values (300,000) Roemer's value was much too low. It was, however, great enough to strain the credulity of many of his contemporaries beyond the breaking point. His work was at first unfavorably received, and it was not until eighteen years after his death that a~expected but conclusive confirmation of his work put an end to the current doubts. Before this description of Roemer's determination of the speed of light ends, it is worth noticing that it involves the principle of the Doppler effect, already studied in the chapters on sound (page 355). In this case, as with sound, the apparent period of a regularly recurrent event is altered by the motion of the observer toward and away from the source. The Doppler relation for the case of a moving observer applies, viz.;

n'=n(1 ± ~}

(1)

Knowing the period (reciprocal of the frequency n) of the satellite and the speed v of the earth in its orbit, and having measured the apparent period, one can immediately calculate the speed V of light by equation (1).

402

LIGHT

Accurate Measurement of the Speed of Light Of the work of Bradley, whose observations furnished the crucial verification and correction of Roemer's results, as well as of the great refinements introduced into the measurement of the speed of light successively by Fizeau, Foucault, and Michelson, space will allow but scant account. The method of Fizeau (165 :340) is perhaps the most significant, partly because it furnished the model followed by the succeeding experimenters, but also because it embodied in an effective experimental method the principle which Galileo had unsuccessfully tried. Fizeau replaced Galileo's first lantern shutter by a rapidly revolving toothed disk, the successive apertures and teeth of which allowed some nine thousand pulses of light to pass each second. He replaced Galile~'s second lantern by a mirror placed about five miles away, which reflected these light pulses back to the toothed disk. If the disk was stationary, the light could be made to return through the same aperture that it had traversed in the outward journey. But if the disk was turning fast enough, the path of the returning pulse of light would be blocked by the tooth adjacent to the original aperture, which in the meantime had moved into the path of the beam. If the speed of the disk was doubled, the light could again return, but now through the adjacent aperture, not through the original one. Calculation of the time which the light required to make the double journey was made possible by noting the speed of the disk and the number of teeth; from these the speed of light could be computed. The value thus obtained was about 5 per cent higher than that commonly accepted now. Thirteen years later, in 1862, Leon Foucault avoided some of the inaccuracies which were inescapable in Fizeau's method, by substituting a rotating mirror for the toothed wheel and thus secured greater precision of results. By further improvement of technique, Michelson, in 1880 and again in 1927, secured the highest accuracy thus far attained. The final value was: 299,796 ± 4 km./sec.

Some of the Implications of the High Speed of Light Probably more attention has been given to precise measurement of the speed of light than has been given to the determination of any other natural constant. This is occasioned by the fact that the speed of light enters as a fundamental element into so many aspects of physics. Once determined, it applies not only to all visible light, as well as to the ultra-violet and infrared regions, but to all other types of electromagnetic radiation, from radio signals to X-rays. It will be found in the theory of electricity and magnetism as a ratio of units not obviously associated with light at all.

ELEMENTARY PROPERTIES OF LIGHT

403

Also, it plays a central role in the theory of relativity, where it appears as the upper limiting value of the speed that any object may attain. But one of the first effects of the discovery of the huge speed of light was to set the pattern for speculations about the nature of the so-called ether. Especially when more accurate values of the speed of light became available through Bradley in 1729, sixty years later, and through Fizeau and Foucault, nearly two hundred years later, did physicists begin to proclaim that the ether was becoming as well known as the other entities of physics. Indeed some nineteenth-century physicists expressed more confidence in the objective existence of ether than of matter. The speed v of a wave in an elastic medium is related to its elasticity E and its density d by the expression (cf. page 331)

v={f

(2)

If the speed characteristic of waves in an otherwise unknown medium, such as the ether, is known, application of the above equation yields the numerical ratio

(!) of the elasticity to the density.

If an approximate value of

either of these constants is known, the value of the other becomes calculable to a like degree of accuracy. Now the facts that the ether was not able to make itself evident to the senses and that the members of the solar system moved through it without encountering any measurable resistance (see page 163) indicated that the density of the ether must be phenomenally low, as compared with that of any other known substance. And yet it appeared that, notwithstanding such tenuity, ether must possess a very great rigidity. For, substituting in equation (1) the highest density consistent with the failure of the ether to make itself evident to human senses, one finds that E assumes a value comparable to that of steel. These characteristics, fantastically low density and abnormally high rigidity, were hard to reconcile, but there was no escape as long as light was pictured as a wave motion in an elastic medium.

Speed of Light in Other Media than Air All this was an outgrowth of the measurement of the speed of light in air. The study of the speed of light in other media had some even more far reaching, though less fanciful, outgrowths. Whether it was greater in such media than it was in air, or less, was a moot question until as late as 1850 when Foucault, in his early experiments with the rotating mirror method demonstrated for the first time that" the velocity of light is less in water than in air." 1 This was twelve years before he made his precision measurement of the speed of light in air. Dur1

Comptes Rendus, 30, 556 (1850).

404

LIGHT

ing the seven teen th and eighteen th centuries the prevailing opinion had been that light traveled more rapidly in dense media than in air. Such was the opinion, already referred to, of Descartes, whose following constituted a philosophical cult almost as undiscriminating as the Scholastics' cult of Aristotle. Newton also had cast the weight of his opinion on that side, under the impression that the refraction of light could best be accounted for in this way (190:226-28; 189:79). Possibly this opinion received some support by analogy with the known fact that sound traveled more rapidly in metals than in air. In any case, the contention of Huyger.s in 1678 (139:32-38) that light must slow down upon entering dense media was almost entirely disregarded for a century.

Refraction of Light There was as much opportunity for difference of opinion concerning the change in direction that characterized the passage of light in to a dense medium, as there was concerning the change in speed. Aristotle, in his Book of Problems, correctly described the appearance of an oar dipped in water. Ptolemy (ca. 127-51 A.D.) tabulated angles of refraction corresponding to angles of incidence for air into water, air into a piece of glass, and water into glass and sought for some law connecting these angles. He concluded that the ratio of the angle of incidence to that of refraction was constant for the same pair of media, a generalization which seems scarcely justified on the basis of his tables of refraction. PTOLEMY'S TABLE OF REFRACTIONS

(274 :80) Angle of Incidence

Air to Water

Air to Glass

10 20

7

9!

13! 20! 25

18! 27

40

8 15! 22! 28

50

35

30

60

40! 45

34! 38!

50

42

30

70 80

Water to Glass

35 42! 49! 56 62

Alhazen (965-ca. 1039) - an Arabian investigator, whose book on optics had probably a greater effect on Western thought than any other single work in this field, except possibly Newton's Opticks - pointed out the error of Ptolemy's generalization. Alhazen's own positive contribution was that of being the first to state that the reflected or refracted ray lay in the same plane with the incident ray and that this plane was always perpendicular to the plane dividing the two media.

ELEMENTARY PROPERTIES OF LIGHT

405

FIG. 215.

ALHAZEN DEMONSTRATING REFRACTION (Courtesy of Bausch and Lomb Optical Company.)

Ptolemy's tables were extended (though not entirely correctly) by Vitellio about 1270 and by Kircher (1601-80). Kepler, who, in addition to his astronomical studies, made important contributions to optics, was so impressed with the work of the former that he entitled his first book on optics (1604) A Supplement to Vitellio. He started his second book on optics, Dioptrice, written seven years later, by describing his experiments on refraction, in which he used the device illustrated herewith (Fig. 216). He, like Ptolemy, tried to discover some relation between the angles of incidence and refraction. The best he could do was to record that, for angles of incidence less than about 30° in air, the angle of refraction in glass was

406

LIGHT

about two thirds the angle of incidence, a formulation not greatly in advance of that of Ptolemy, nearly 1500 years before. It was natural for these early investigators to look for a relation between the angles, but in 1621 Willebrord Snell (1591-1626) discovered that the behavior of light upon refraction could be formulated, not by a ratio of the angles involved, but instead by a ratio of trigonometric functions of the angles. Snell's law,as it is now stated, is: FIG. 216.

KEPLER'S REFRACTOMETER (From his DioPtrice of 1611.)

sm i sm r

-.-=µ

(3)

where i represents the angle of incidence, r the angle of refraction, and µ the ratio of the two, termed the index of refraction of the second medium. Snell's original formulation was somewhat different from this. It was put into the above form by Descartes. There is reason to think that Descartes had seen Snell's manuscript, though he subsequently published the law of refraction in the above form as his own discovery.

Controversial Outgrowth of Refraction Observations But the concept of index of refraction was destined to exhibit a far deeper significance than that of being merely a name for the ratio of the sines of the two angles involved. In a paper presented to the Royal Academy of Sciences at Paris in 1678, a paper which is justly famous, Huygens propounded the theory that the ratio of the sines, which appears in equation (3) above, was also the ratio of the speeds of light in the two media, the speed in the less dense medium such as air, being always the greater of the two (139:38). At the time that this theory was propounded there was no possible way of adducing an experimental verification. This was a great misfortune. Newton, whose theory first appeared in his Principia nine years later, agreed with Huygens in associating the ratio of the sines with the ratio of the speeds of light, but specified, contrary to Huygens, that the speed in the less dense medium (that is, air) was to be considered the lower of the two speeds. In other words, he insisted that light speeded up when it entered glass from air, whereas Huygens had specified that it would be found to slow down. There was thus a clear-cut issue between these two views which could be decided only by experiment. But the required experimental technique was not forthcoming for nearly a century and three quarters, as has already been observed. In the meantime the enormous prestige of Newton carried the day, and his incorrect opinion about the relative speeds of light took precedence during the entire eighteenth century over the correct opinion of Huygens.

ELEMENTARY PROPERTIES OF LIGHT

407

FIG. 217. REFRACTION OF RIPPLES (From General Physics for Colleges, by Webster, Farwell and Drew. The Century Company, publishers.)

A simple illustration of refraction produced by the diminution of the speed of a wave is to be found in the change in direction of waves or ripples upon oblique approach to a beach. Fig. 217 shows this effect on a train of ripples. The water, very shallow everywhere, is rendered still more shallow in the lower portion of the photograph by a sheet of glass laid on the bottom of the tray. The resulting diminution in the speed of the lower portion of the ripples produces the refraction shown by the photograph. This is also true, we now know, with light waves entering such substances as glass or water. Newton, however, reasoned along quite a different line. He considered that the behavior of light, upon oblique entrance to a medium that was optically more dense, indicated that the particles constituting the light were subject to an attractive force as they FIG. 218. REFRACTION OF LIGHT approached the new medium. This force, he reasoned, bent the particles out of their previous course in the way to be expected fro111 such attraction. Hence, he was confident that their speeds in the second medium would be found greater than in the first by an amount deducible from a process of composition of velocities ( 190: 22628; 189: 79). Fig. 219 illustrates this concept. The original velocity in air is represented, both as to direction and speed by the vector OA. The velocity of the light after it has entered the glass is represented, again both as to direction and speed, by the vector OG. The vector OB represents the velocity, superposed by the attractive action of the glass on that possessed by the approaching corpuscles of light, which, when compounded with OA pro-

408

LIGHT

duces OG. Any demonstration that the speed represented by OG was, by actual measurement, less than that represented by OA, would, of course, completely invalidate this simple and attrac0 tive theory of the nature of the refraction of light and give support to the wave hypothesis, which requires the velocity of light to be less in dense media than in air. Not until 1850, however, was the demonstration made that the speed of light was less in an A optically dense medium than in air, instead of B \ greater as had been supposed. Scientific doc\ \ trine, like other forms of group opinion, ad\\ vances - or retrogrades - by surges. The \ history of optics in the nineteen th century was \ \ characterized by a surge of opinion in favor of ~ G the wave-hypothesis of light which carried everything before it as relentlessly as the surge FIG. 219. NEWTON INVOKED THE PRINCIPLE OF COMPOof opposite opinion had done in the eighteen th. SITION OF FORCES TO ACThough the volume of experimental evidence COUNT FOR THE REFRACwhich accumulated in support of the wave TION OF LIGHT hypothesis was very impressive, it was perhaps not so much a cause as a result of the new conviction that light consisted of waves. In the sciences, as in other types of human endeavor, men are more likely to discover what they are seeking than what they do not wish to find.

'

Refraction and Scientific Doctrine The growth of knowledge about refraction provides an excellent example of the development of the general body of scientific knowledge. The first step is the replacement of the casual qualitative observations of the ancients by attempts at systematic measurement, as Ptolemy did in measuring angles of refraction. These first quantitive observations are later sometimes much later - refined and extended; in the course of this some errors not infrequently creep in, as they did at the hands of Vitellio. At the same time a painstaking search is conducted for a general law, which the mass of accumulated data seems to exemplify. This is often more a game of blindman's buff than anything else, but one in which it is perfectly fair to '' peek'' if you can get the bandage off. The flash of insight that follows often bridges centuries of groping, as did Snell's discovery that trigonometric functions constituted the key to a law of refraction, instead of angles themselves. Unfortunately, unethical practices in the pursuit of personal ambition are not entirely unheard of in the history of science, an example of which is to be found, doubtless with mitigating circumstances, in Descartes' plagiarism upon Snell.

ELEMENTARY PROPERTIES OF LIGHT

409

But the amassing of data and their crystallization into formulas is only the first stage of scientific progress. Theoretical interpretation must follow. Here the clash of varying senses of validity has its full play. This stage is usually a battle of giants, for instance, Huygens and Newton, the principal outcome of the battle often being reformulations of the problem, which may still conflict but which are in terms that lend themselves to crucial experiment. Not infrequently this crucial experiment is long in coming, as was Foucault's experiment of 1850; and during the interval men of science are more likely to exhibit the human trait of dogmatic adherence to one side or the other, as they did in accepting the opinion of Newton, than they are to adopt the more tenable scientific position, that of suspended judgment. Sometimes, also, the "crucial" experiment turns out to be not so crucial as had been supposed; and science must be willing, when circumstances seem to require it, to reopen questions supposedly settled once and for all - as is occurring now in the twentieth century with regard to the nature of light.

Problems on Chapter 29 1. In a repetition of Fizeau's determination of the speed of light the distance between the wheel and mirror was 8500 meters. The wheel had 88 teeth. When the first occultation of the reflected beam occurred the wheel was rotating at 100 r. p. s. What value did this give for the speed of light? 3 X 108 meters/sec.

2. Roemer observed that the lapse of time between successive occultations of one of Jupiter's moons was seven minutes greater when the earth was receding from Jupiter than when it was approaching. The approximate period of the satellite being 42} hours: what was the speed of light given by these observations? Take the radius of the earth's orbit as 1.5 X 1011 meters. 2.2 X 108 meters/sec. 3. If, due to atmospheric refraction, the sun is visible for 15 minutes after it has actually set, what is its angular displacement? 4 degrees. 4. Assuming the rigidity of the ether to be approximately that of steel (E = 8 X 1010 newtons/sq. meter), what is the density of the ether in kg. per sq. meter? 10-1• 5. What percentages of error were involved in the second column of Ptolemy's table of refraction if the index of refraction of water is {? Errors up to 7 per cent.

I have seen the whole book which Snellius wrote upon this subject, though not yet published; and am told that Descartes saw it too; and perhaps it was from hence he found that the true measure of refractions was to be taken from the sines of the angles. ROBERT SMITH, 1736

CHAPTER30

Illumination And he lighted the lamps. EXODUS 40 :25

Light in Human Economy I always have supper for him when he gets home, and lights, lights, plenty of lights - Dizzy always likes lights; and then he tells me everything that has happened in the House, and then I clap him off to bed.

Lady Beaconsfield told, nearly a century ago, how she helped Lord Beaconsfield, Benjamin Disraeli, Prime Minister of England, guide the destinies of the British Empire through one of its most difficult periods. In the same sentence she identified what was - and still is - one of the principal contributors to human morale: "light, plenty of light." The development of artificial illumination has been at least as notable as have the other products of applied science. Though it does not seem to have made as sensational an appeal as transportation and communication have, it has contributed its full share to the modern pattern of human behavior. The technological problems whose solutions have been required have indeed been fully as difficult as those in other types of applied science. THUS

Ancient and Modern Standards of Illumination The most ancient illuminant which has survived is the torch. A number of torches identified as relics of the Stone Age, about 100,000 years ago, were unearthed in 1870 (134:199). It is scarcely an exaggeration to say that until less than one hundred years ago, when kerosene became available in quantity, humanity remained in the torch age, for neither the ancient pottery lamps nor the later candles furnished more light than did torches if as much. It is true that gas as an illuminant was known and used experimentally fifty years before that. But at that time scarcely more light came from a gas burner than from a torch. The chief advantage of early gas illumination was convenience in such projects as the lighting of city streets. The first American cities so illuminated were Baltimore, Boston, and New York, in the order named, during the eighteen-twenties. Until very recently artificial illumination was confined to providing the minimum amount of light that would enable man to continue the more es-

ILLUMINATION

41 I

sential activities normally carried on during the day. Artificial illumination competed merely with darkness, never with daylight. To imitate daylight, either in quality or quantity, was at first impossible and later prohibitively expensive on any scale large enough to be generally useful. Within the last decade, however, new types of il_lyminant have provided a quality scarcely distinguishable from north-sky light, the best for general illumination. A campaign of encouragement has in addition brought many users to the point of providing a quantity of illumination rivaling and occasionally even exceeding that of daylight. This encouragement has not been entirely disinterested, to be sure, for the distributors of electric power have much to gain through general acceptance of the new standards of illumination. But the public also has much to gain, since the previous "starvation diet" of light has been taking its toll in inefficiency, accidents, and nervous ailments. There is scarcely a possibility that the public can be persuaded to provide more illumination than is good for it. In the meantime sound inducements for more illumination are being offered in this country in the form of electric rates which are much lower than those anywhere else in the world, as well as in improved lighting equipment at progressively lower cost. This form of propaganda by "the Utilities" need never cause any misgiving.

Luminous Intensity and Its Unit Sources of "artificial" light vary greatly in their intensity. From night lamps, the luminous equivalents of a single candle, to beacon lights, whose luminous intensities may be expressed as the equivalent of millions of candles, the whole range of human requirements is covered. Moreover, a given source will invariably appear to be of different intensities when viewed from different angles. This effect is enhanced by reflectors designed to distribute light to the best advantage and becomes especially pronounced for such devices as headlights and searchlights. When the luminous intensity of a source is specified without qualification, it is either the average of all directions (mean spherical candlepower) or is limited by the context to a specific direction. One of the few anachronisms of scientific terminology is the use of the term candlepower to describe the unit of luminous intensity. Only the name is anachronistic, however. The original unit was a candle, as the name indicates, of a particular kind, burning at a specified rate. But the most recent definition of the unit is a product of recent international agreement between England, France, and the United States, effective in 1941. This agreement was entered into before the onset of the European war, but there is no compelling reason why it should not be carried out, though for some reason Germany persistently refused to enter into it during the whole course of the negotiations.

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The new internationa"/,,candle is defined as one sixtieth of the luminous intensity provided by one square centimeter of a "black body" at the temperature of melting platinum. 1 The term "black body'· need cause no perplexity. It merely involves the requirement that the nature of the radiating surface shall be such that the surface appears perfectly black when cold. Perfect blackness in the sense of reflecting no visible light is, of course, an unrealizable ideal, but is one which can be approximated pretty closely by various artifices. It is a useful concept in the theory of radiation. In this instance perfect blackness must be invoked because the intensity of an incandescent surface at a given temperature depends somewhat on the degree of blackness of that surface when cold; the blacker it is when cold, the brighter it is when heated to incandescence. Until a few years ago all illuminants were rated in candlepower. The more common sizes of electric light were procurable in 8, 16, or 32 candlepower sizes. Automobile lamp bulbs are still rated that way. All others, however, are rated, not in intensity, but in power consumptioP, that is, not in terms of luminous output but in terms of power input. While incandescent lamps dominated the lighting market this was a fairly convenient way of rating sources of light, for the luminous output was roughly proportional to power input. But with the advent of fluorescent lamps and other new types of illuminant, comparisons of intensity by power ratings have lost all significance. A twenty-watt fluorescent bulb gives several times as much light as a twenty-watt incandescent bulb.

Illumination and Its Units Up to this point, the word "illumination" has been used in its usual loose, general sense. Science has commandeered it, however, as a technical term. As such it gives a numerical measure of the adequacy of the supply of light in the region where it is to be applied. It is, in a sense, of primary interest to the "consumer," whereas intensity is of primary interest to the ''producer.'' After light has left its source it is sometimes used directly, to attract attention, as with electric signs. But in the great majority of cases it is utilized indirectly, by its incidence on a surface, producing what is called technically illumination. Naturally, the illumination of a surface directly facing the source depends both on the I uminous intensity of the source and on its distance from the source, being proportional to the former and inversely proportional to the square of the latter. The inverse square law is already familiar in gravitation and in sound and will appear again in connection with magnetic and electrostatic forces. It depends for its validity, as in the other cases, on the source's being concentrated at a point. 2 Revue de Metrologie Pratique et Legale (2), 18, 10 (1940). An exception to this for gravitation in the case of spherical bodies is treated on page 161. 1 2

ILLUMINATION

413

F1G. 220. Two TYPES OF LIGHT-METER (Courtesy of Illuminating Engineering Society.)

The English unit of illumination is the one most commonly used and is called the foot-candle. As the name indicates, it is the illumination at a surface one foot distant from a source of luminous intensity of one candlepower, the surface being normal to the source. The metric unit is called the lux. It is similarly defined except that a distance of one meter is involved instead of one foot. The ratio of the two units is simply the square of the ratio between the meter and the foot or 10. 76, the foot-candle representing the greater illumination. The lux will be the preferred unit in this chapter. If it becomes necessary to convert an illumination given in luxes to the more common English unit, footcandles, the latter may be approximated by dividing the former by 10 (strictly 10. 76). As has already been intimated, standards of illumination have been rapidly rising. Until recently 50 luxes was considered sufficient illumination for sustained reading. Now 500 or even more are commonly provided, and the requirements are still rising. That this is not excessive is indicated by the fact that daylight illumination on the north side of a building on an

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average bright day is about 5000 luxes and in direct sunlight is about 100,000. Measurements of illumination are commonly made by the "photovoltaic" type of light-meter, illustrated in Fig. 220. They are more fully described on page 743 than is feasible here. They depend for their action on the fact that when certain "photo-voltaic" materials are illuminated, they generate minute amounts of electric power. These tiny power impulses are made to energize electric meters, graduated to read in luxes or, more commonly, foot-candles.

Luminous Flux and Its Unit The fact that the intensity of a source of light, measured in candlepower, may be quite different in different directions has already been mentioned. For some purposes the total amount of light emitted by an illuminant, the "flow" or flux, is of importance. Sometimes the intensity of a lamp is given in mean spherical candlepower, that is, the average of the intensities in all directions. In such a case the total flux could be said to be 4 1r times the mean spherical candlepower. The factor 41r originates in the geometry of the sphere about the lamp as a center, and over which the illumination is supposed to be spread. The area of a sphere is 4 1r times the square of the radius. It is natural to take as the unit of luminous flux the flux from a source of unit mean spherical candlepower through one square meter of the surface of a sphere of one meter radius having the lamp at its center .1 This is called a lumen. The area of the en tire sphere wil I be 4 1r times the area through FIG. 221. LUMINOUS FLUX OVER UNIT which the unit flux, as thus deSoun ANGLE fined, passes. Consequently the total flux from a lamp of one mean spherical candlepower is 4 1r lumens. The shortcomings of wattage as a measure of the luminous output of a lamp were noted above. It would be much more sensible to use luminous flux in this role. Indeed it would be eminently appropriate to do so. The present uninformative ratings of, say, two twenty-watt lamps, one incandescent and one fluorescent, would then be replaced by their flux 1

Though the lumen is defined here as a metric unit, there is no distinction between this and the English unit. The flux through one square foot of a sphere of one foot radius having the lamp as its center will obviously be the same as the flux described abve. See Fig. 221.

ILLUMINATION

FIG.

415

222. PHOTOGRAPH OF A REPLICA OF EDISON'S FIRST SUCCESSFUL INCANDESCENT LAMP (Courtesy of General Electric Company.)

ratings of 240 lumens and 800 Iumens respectively. Unfortunately there is no prospect that such a change will occur in the near future. There is, however, one respect in which the lumen is coming into common use. That is in rating the luminous efficiencies of lamps. Since the proper unit of luminous output is the lumen and of power input is the watt, the luminous efficiency of electric lamps is quite appropriately being specified in lumens per watt. There has been a steady increase in the efficiency of electric illumination since the time of Edison's first incandescent lamp, illustrated in Fig. 222. That lamp gave 1.4 lumens per watt. Tungsten filament lamps of comparable size give 12 lumens per watt and the new fluorescent lamps give 40. It is rather curious that the lumen, denied access to common use up to the present through lamp output ratings, seems to be entering by the back door of efficiency ratings.

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LIGHT

The Development of Electric Illumination Electric illumination, a many-sided development, has had an interesting history. The earliest successful ventures will be described as part of the story of electricity (page 623). Those occurred in the first years of the nineteenth century, but not until the last quarter of that century did electric illumination become a widespread possibility. This came about when two conditions necessary for its success were finally met. One was the major reduction in cost of electrical energy effected by the perfection of the electric generator. The other was the invention of small electric lamps. The electric arc with which Davey and others pioneered in the early eighteenhundreds had huge outputs (several thousand lumens each) and could not be reduced materially in size. They could be used for street lighting and for large auditoriums, but not for the lighting of residences. The difficulty was met by the perfection of the incandescent lamp by Edison in 1879. The subsequent two generations have seen steady improvements, the two most notable developments being the substitution of the tungsten for the "carbon" filament in 1907 and the advent of the fluorescent lamp in 1939. Each development involved both a marked improvement in the quality of the light and a notable increase in efficiency. In the fluorescent lamp the substantial equivalent of daylight quality is attained, though certain colored varieties are more efficient than the "daylight" type. There is, however, still room for increase in efficiency. Only about 3 per cent of the power consumption of the tungsten incandescent lamp is converted into light and only about 11 per cent in the case of the fluorescent lamp. Fluorescent lamps have come into use so recently that a brief description of them may be appropriate. Unlike most other illuminants, they do not depend on high temperature for their luminosity. The luminosity results from the fluorescence of certain powders coated over the interior of the long glass tubes characteristic of these lamps. The fluorescence is induced by the ultra-violet radiation attending an electrical discharge through lowpressure mercury vapor within the tube. The color of the light is determined by the chemical composition of the powder coating the interior of the tube. A certain mixture of powders is found to simulate daylight, and this is accordingly the type of fluorescent lamp in greatest demand though it is the least efficient. The lamps are commonly used on alternating current and one of the technical problems that had to be solved in their development arose out of their low power factor (page 718). It is interesting to recall in connection with the electric fluorescent lamp that gas lighting also reached its highest point by taking advantage of fluorescence. Gas "mantles," the central feature of the so-called "Welsbach" burners of a generation ago, were merely fluorescent screens the characteristic luminescence of which was induced when they were subjected to the temperature of the gas flame. Welsbach burners yielded as

ILLUMINATION

417

much as six times the light which the gas itself was capable of producing. As is the case with the electric fluorescent lamp, the quality of light was even more notable than the quantity, the whiteness being more pronounced than in any other illuminant except the electric arc until very recent years. The inconvenience and fragility of the Welsbach burner, coupled with its high consumption of oxygen, kept it from being a serious competitor with electric illumination in spite of its superior quality.

Comparison of Luminous Intensities Light flux is a form of power in the technical meaning of that term. Physicists would find their aesthetic sense gratified if only the practice had been established of measuring light in watts as mechanical and electrical power are measured and as rate of flow of sound energy is sometimes specified. But as with rates of energy flow in the form of heat, an arbitrary unit of light flux holds the field. For both heat and light the arbitrary basic units were established before their numerical relations with the absolute unit of power were known or, indeed, even imagined. As with luminous flux, so with luminous intensity. Intensity, being simply flux per unit area, might well be specified in absolute units as watts per square meter instead of in candlepower. Since common practice does not sanction the absolute unit, intensities may only be compared with each other in terms of the purely arbitrary unit. The procedure of comparing intensities of sources is called photometry (" light measurement"), and the instruments commonly used for the purpose are photometers. Until recently, almost all photometers involved the visual matching of illuminated areas. Though a human observer is very unreliable in any direct comparison of different intensities, he is fairly consistent in the ability to tell when two identical adjacent surfaces are of the same brightness when illuminated from the same kind of source. Visual photometers invariably utilize this ability, differing among themselves principally in the means taken to adjust to equality the illumination from the two sources. The most common arrangement is to place the two sources at different distances until a match is secured. Another is to interpose a rotating sectored wheel in the path of the more intense illumination, adjusting the proportion of the open to the closed sectors until a match is secured. Still another way is by the use of polarizers such as those described in the chapter on polarized light. The central feature in any visual-match type of photometer is the optical device for juxtaposing the illuminated surfaces so that both can be seen simultaneously. One arrangement for doing this, the so-called Lummer-Brodhun photometer head, is illustrated in Fig. 223. The two lights being compared are off the scene, at measurable distances to right and left. A system of reflecting prisms in the central box enables the observer, upon applying his eye to the small oblique observing telescope, to

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LIGHT

FIG. 223. A

COMMON TYPE OF PHOTOMETER HEAD OF A VISUAL-MATCH TYPE

see the opposite sides of the white screen receiving the light from the respective lamps, apparently side by side. The carriage is moved until a visual match is secured. Then application of the inverse square law - a questionable procedure unless the sources are small in comparison with the distances involved - enables the ratio of the intensities of the two sources in the given direction to be deduced. The visual-match types of photometer have been rendered somewhat obsolete, however, by the photovoltaic type of light meter already mentioned. Comparison of the pointer readings when the light meter is exposed to two illuminan ts successively, at the same distance, gives a direct measure of the relative intensities of the two sources. No adjustments to effect a match are required, as in visual photometers. The replacement of the old visual-match types of photometer by the photovoltaic type brings its own problems, however. The chief problem is manufacturing photovoltaic surfaces which possess the same sensitivities as the human eye to the various colors making up the light being measured. The difficulty in solving that problem delayed the advent of the photovoltaic photometer for several years.

ILLUMINATION

419

FIG. 224. ULBRICHT SPHERE, OPEN TO CHANGE THE LAMP (From Measurement of Radiant Energy, W. E. Forsythe, editor. McGraw-Hill Company, publishers.}

Comparison of Luminous Fluxes The fact has already been pointed out that the usual illuminant c!istributes its flux quite non-uniformly. Its candlepower, therefore, is quite different when measured from different directions. It is desirable frequently to know the total flux emitted by a lamp, regardless of how that flux may be distributed. The preceding statements of luminous efficiencies of various types of lamp involve total flux rather than candlepower. The ordinary photometer, whether of the visual or the photovoltaic type, is not capable of yielding such information directly. In the purely theoretical case of a uniformly distributed flux, the candlepower in any direction multiplied by 4 1r (the area of the sphere of unit radius having the lamp at its center) yields the total flux. But since uniform distribution practically never obtains, no relation can be specified between the candlepower and the total flux from a given lamp. The value of the flux can be approximated in such a case by measuring the candlepower in a sufficient number of properly chosen directions, striking an average and then proceeding as in the case of uniform distribution of flux. The indirection involved in the approximation just described may be

420

LIGHT

avoided by the use of the so-called Ulbricht sphere. This is a hollow sphere, of large diameter in comparison with the longest dimension of the lamp being tested, coated on the inside with a special variety of highly diffusing white paint, and having set into it at one point a window of translucent glass. The lamp to be measured is placed inside of the sphere and between it and the window is placed a screen so that the direct rays from the lamp do not strike the window. The theory of the device shows that the brightness of the window is then directly proportional to the 1uminous flux of the lamp, provided that certain precautions are observed.

Standard Lamps Whatever type of photometer is used, the intensity or the flux of a lamp under test becomes known only if that to which it is being compared is known. The basic standard, the "new international candle," has already been described. I ts use is, however, necessarily confined to the principal standardizing laboratories. The common standard lamp is a specified variety of incandescent electric light, duly aged so that it has settled down to a constant performance, carefully guarded against mechanical and electrical misuse, and operated at a specified voltage. Such lamps, calibrated in terms of a basic standard, are securable from government standardizing agencies, in this country the United States Bureau of Standards.

The Automobile Headlight Next to interior illumination, highway lighting is the most urgent requirement, at least in the United States. This is provided largely by flooding the roads ahead of the motor cars by lights on the cars themselves. Though this is far from an ideal solution of the problem of highway illumination, it is the only one feasible at present outside of populous areas. When adequate illumination is provided in this way, however, the glare is intolerable to approaching drivers. The central problem of automobile headlighting is how to get sufficient illumination without glare. A solution to this problem which is in sight through the agency of polarized light is described in Chapter 38. It cannot be effected immediately, however, and in the meantime the less complete method now available is worthy of attention. Its principal defect is that it depends upon the initiative of drivers in momentarily pointing their headlight beams slightly downward out of consideration for the plight of other drivers approaching. Though only a simple flick of a conveniently located switch is involved, a disheartening proportion of those using the road are notably uncooperative in this respect. The point of departure for automobile headlighting, as for other types of light projection, is the formation of a parallel beam. The standard device

ILLUMINATION

42 I

for converting the divergent light from a concentrated source into a substantially parallel beam in this connection is the so-called paraboloidal reflector. The principle is developed on page 463. For headlighting purposes the reflected beam from such a reflector is modified by its passage through an appropriately contoured cover glass. The beam is spread laterally by vertical flutes, and distributed vertically by small-angled prisms cast into the glass. The depression of the beam for the convenience of approaching motorists is effected by a second filament in the headlight bulb, located slightly above the main filament. Fig. 225 shows how this acts. It represents the vertical mid-section of a point source and paraboloidal reflector. The dotted lines show the normals to the surface at the points of incidence. The fact that the angle of reflection has the same value as the angle of incidence in each case will be evident. Some headlamps use the same artifice to FIG. 225. THE EFFECT OF deflect the beam to the right. To produce DISPLACING THE SOURCE ON A BEAM REFLECTED FROM A this, the second filament is mounted at the PARABOLCID left of the main filament, as viewed from the driver's seat. Figure 225 may also be used to illustrate this effect by regarding it as a horizontal section instead of a vertical. Horizontal deflection, in headlamps which incorporate it, is invariably combined with depression. The second filament in these lamps is thus above and to the left of the main filament viewed from the rear.

Problems on Chapter 30 1. A 2-meter photometer of the intensity-matching variety shows equal illuminations of the screen when the lamps being compared are 75 ems. and 125 ems. distant respectively. If the 20-candlepower standard is the nearer of the two lamps, what is the candlepower of the other? Why would this type of photometer not give accurate results for frosted lamps? 56 c.p. 2. On a line through two electric lights of candlepower 60 and 100 respectively, 10 feet apart, is a screen. Find the point between them where the two sides of the screen will have the same intensity of illumination. Find also the point, not between the two, where the illumination of one side of the screen is the same as that of both sides before. 5.6 and 44 ft. from the larger. 3. Three clerks have their desks side by side, 4 ft. between centers. Eight feet above the center desk is a 500-c.p. lamp. What is the illumination at the center of each desk, taking obliquity into account? 7.8, 5.6 foot-candles. 4. If the desks of problem 3 are 4 ft. X

2-l ft., how many lumens does each desk re-

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ceive, assuming the intensity at the center to be the average in each case? Does this assumption possess an equal degree of validity for each desk? Assume also that the flux from the lamp is uniformly distributed. 9.8 and 5.6

5. A 100-watt incandescent lamp of the most efficient type delivers about 1200 lumens. The power equivalent of one lumen is about .0093 watt. efficiency of the lamp?

What is the 11 per cent

The history of science shows that the greatest advances have always been made by men who undertook their inquiries into Nature without thought of proximate or ultimate practical application or pecuniary reward. RICHARD GREGORY,

1900

CHAPTER31 Image Formation Glasses can be so shaped that things very far away appear very near and vice versa; so that from an incredible distance we may read the smallest letters and may count things, however small. ROGER BACON, thirteenth century

Image Formation in Daily Experience of the daily utility of light is associated with the formation of what are termed images. By far the larger part of this image formation is performed by the eye. The usefulness of the eye is greatly extended by spectacles, telescopes, microscopes, cameras, and picture projectors, all of them image-forming instruments. Such utilitarian devices, though not the primary concern of pure science, are natural by-products. If not considered too narrowly, they often furnish worthy material for scientific study, being themselves the outgrowth of extensive scientific research and development.

MOST

The Pinhole as an Image-Forming Device Common experience with photography and with picture projection, as in the cinema, has created an impression that for the production of images a

FIG. 226. A PINHOLE PHOTOGRAPH

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LIGHT

complicated and more or less expensive equipment of lenses and optical accessories is required. While it is true that image-production is greatly facilitated by these devices, they are by no means indispensable. Faithful images may be formed, even photographs taken, with equipment no more pretentious than a diaphragm pierced with a small pinhole, backed by some sort of box or dark chamber (camera obscura) to block out stray light. Fig. 226 is a photograph taken in this way. The great disadvantage in this type of photography is the faintness of the image, with consequent great length of exposure required. The accompanying photograph required twenty minutes' exposure in full sunlight. The first observation of image formation on record is Aristotle's remark (19: 1: 333) that when the light of the sun passed through a square hole, it produced a circular illuminated space on a screen at a little distance. Aristotle carried this observation so far as to remark that this circular patch of light became a crescent when the experiment was performed at the

FIG. 227. MAUROLYCUS OF MESSINA (From The Photismi de Lumine of Maurolycus, translated by Henry Crew. The Macmillan Company, publishers.)

IMAGE FORMATION

425

time of a partial eclipse, the crescent being reversed in position with reference to that of the partially eclipsed sun. In view of the remarkable acuteness of this observation it is amazing that Aristotle did not stumble onto a correct explanation. But neither he nor any other investigator was able to formulate a tenable hypothesis until Maurolycus (1494-1577) in 1575 hit upon the correct explanation. He said (172: 29-30): When the sun is shining through any aperture, the further the rays proceed beyond the aperture so much more does the light, which is projected upon a plane paral1el to the circle, which bounds the radiating surface, approach the form of this circle; and finally, the senses being deceived, appears to be a perfect circle, since every aperture used for the transmission of solar rays is, with respect to the size and distance of the sun, a negligible quantity .... A source of light therefore radiating through any aperture, produces an infinity of pyramids which (as the light leaves the aperture) ... gradually unite into one; and finally all become a single pyramid.

Kepler's Explanation But perhaps the most painstaking of the earlier explanations of this phenomenon was made by Kepler, to whom we are indebted for some of the most fundamental contributions in this field. Quite independently of Maurolycus, Kepler showed geometrically that a rectangular object would ' ' -------, I I I I produce a rectangular image, even if II I I I I I I the aperture forming the image were II I I I triangular, and vice versa. Fig. 228 shows his drawings. His method of I I demonstration was rather ingenious. II I I To one point of the object (that is, I I ' a rectangle) he attached a thread and I I I passed it through the triangular open- I~' I I I ing. By sliding the taut thread along I I I I I the edge of this opening, he traced a IJ I I triangle on the opposite wall. By attaching the thread successively to different points of the rectangular object and repeating the foregoing FIG. 228. KEPLER'S EXPLANATION OF process, the different, partially overPINHOLE IMAGE FORMATION lapping triangles on the wall produced an imperfect image of the rectangle. If the triangular aperture were large, the imperfections would be so prominent as virtually to mask the rectangularity of the image, but if the size of the aperture were reduced, even though retaining its triangular outline, the imperfections could be

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THE EFFECT OF DIMINISHING THE SIZE OF THE APERTURE IN PINHOLE IMAGE FORMATION

correspondingly reduced until the image was a very faithful reproduction of the rectangular object. Fig. 229 shows successive stages in this refinement of the image. It will be evident that in the case of actual image formation, the intensity of the illumination at the image will be in proportion to the area of the aperture. Hence, in this method of image formation, sharpness of the image can be secured only at the expense of reduction in the intensity of illumination. It is for this reason that so long an exposure was required for the photograph from which Fig. 226 was taken. The aperture required for such a photograph is much smaller than any that can actually be produced by the point of a pin or needle. There is, however, a lower limit to the size of an aperture that can be used to advantage. This limit occurs, not only because of deficiency of illumination, but - what is more significant - because as the aperture is diminished beyond a certain point, the imperfections of the image begin to increase instead of decrease, for reasons which will become evident in Chapter 36.

Pinhole Images and Rectilinear Propagation It was specified in the preceding description of Kepler's demonstration that the thread was to be held taut. The optical corollary to this is quite clearly that the light which forms the image shall proceed undeviated through the aperture. That is, the formation of a faithful image by this method is a testimony to the rectilinear propagation of light. It was the accumulated mass of observations of this sort, together with what seemed to be the inescapable conclusion of rectilinear propagation, that formed the principal objection in Newton's mind to the view that light was propagated in some sort of wave motion. If he had known that the imperfections of a pinhole image increase - instead of decrease - when the aperture is reduced beyond a certain point, the course of scientific thought about the

IMAGE FORMATION

427

nature of light would have been very different from what it was for the next century and a half.

The Birth of the Camera Ohscura Though Maurolycus and Kepler share the credit for giving the first correct account of image formation by the pinhole method, they were not the first to make use of the phenomenon - if indeed they made any practical use of it at all. Leonardo da Vinci had presumably used image formation by pinholes as an aid in his sketching more than a century before, for in one of his manuscripts is a drawing of a camera obscura, showing rays of light passing through a pinhole to form an image for the eye to view (234:2:404). He does not, however, describe it nor tell how he used it. The first clear and unequivocal description of such a device occurs in the Magice Naturalis of Giambattista della Porta (1543-1615). In the first edition (1558), written when he was fifteen years old, Porta described a camera obscura, using a pinhole, and stated that it could be used as an aid in sketching by those who would not otherwise possess the ability to do it well (208: 144).

FIG.

230.

GIAMBATTISTA DELLA PORTA

(1543-1615)

428

LIGHT

But in the edition of 1599 (208: 363) Porta told how the camera obscura could be greatly improved by using a converging lens, carefully focused, to form the image in place of the .pinhole. This was an enormous advance. With this modification, Porta invented the modern camera, as far as the working principle of its optical system was concerned, though the chemistry of light-sensitive substances to record the images thus produced was not to come into existence for two and a half centuries. The pinhole variety of camera obscura as a human invention has its prototype in nature. The organ of vision of the nautilus, a lowly sea creature elevated to fame by a poem of Oliver Wendell Holmes, has no lens; FIG. 231. THE EYE OF THE it has simply a tiny aperture, which, acting NAUTILUS; AN EXAMPLE OF as a pinhole, casts a dim image on a rudi''PINHOLE'' IMAGE FORMAmentary retina which covers the rear wall TION of the "dark chamber" or camera obscura which constitutes the eye. In fact, human vision was not understood until it was realized, first by Maurolycus (172: 109) and a few years later by Kepler, that the eye is simply a camera obscura, upon the retina of which the images are cast by the action of the crystalline lens.

The Burning Glass During the same interval that the camera obscura was developing, that is, from the time of the early Greeks up to the beginning of the scientific era, the burning glass was making intermittent appeals to the imagination of the curious. Reference has already been made to its appearance in Aristophanes' Clouds of the fifth century B.C. Of the many later users of burning glasses it is not necessary to give an account. Pliny (199:6:382) mentions the use of spherical vessels filled with water to produce the effect. The description is inaccurate and apparently from hearsay, for he states that the glass vessels themselves become heated in the process. Numerous spherical and lens-shaped objects of glass excavated from ancient graves attest the common use of burning glasses, presumably to ignite fuel. What none of the ancient users comprehended and what the modern layman probably comprehends as little is that these glasses, whether acting by reflection or refraction, produced at their foci, not mere points of concentrated light and heat, but images of the sun, nonetheless images in that they were very imperfect. The failure to realize this fact delayed the development of optical science for many centuries. For if it had been realized, such glasses would have been put to use in forming the images of

IMAGE FORMATION

429

other objects in addition to that of the sun, and the telescope and microscope would have developed long before they did.

Roger Bacon and His Concave Mirrors The first man who seems to have recognized that burning glasses were really image-forming devices and to have made such uses of that fact as would naturally suggest themselves, was Roger Bacon (1216-94). Bacon states that aided by his friend, the famous Petrus Peregrinus, 1 he spent three years of labor making just one concave mirror (25 :1 :116; 24:46, 112, 116), but that he made better ones at less expense afterward, profiting by this experience. Unfortunately, we have no direct information about what he did with these mirrors after they were completed. It is certain that he did not expend all that effort merely to make burning glasses. The point is worth pursuing, for concave mirrors have played a large part in the design of telescopes, and the time commonly assigned to the first construction of such a telescope is more than three centuries after the time of Roger Bacon. In 1385, about a century after Bacon's time, one Peter of Trau records that 2 Friar Roger, called Bachon ... made two mirrors in the University of Oxford: by one of them you could light a candle at any hour, day or night; in the other you could see what people were doing in any part of the world.

Peter goes on to say that the students spent so much time diverting themselves with these mirrors, to the detriment of their studies, that the mirrors were broken by order of the University authorities. Pedagogical difficulties with extra-curricular activities are apparently not confined to modern times. Two centuries later Robert Recorde, court physician to Edward VI, wrote (220: preface): Great talke there is of a glasse that he (fryer Bakon) made in Oxforde, in which men myght see thynges that were the above simile, if a mass hung on 0:: the end of a spring were immersed 10 20 30 40 in heavy oil, its oscillations would be damped more rapidly than in a FIG. 469. Two INSTANCES OF CoNlight liquid or in air. If the oil DENSER DISCHARGE IN CIRCUITS HAVING GREATER THAN THE CRITICAL DAMPING were extremely viscous - as mo- RESISTANCE lasses is - no oscillations would occur at all. The corresponding electrical case was identified in Kelvin's theory, the critical damping resistance being the minimum resistance required to inhibit all oscillation. Two representative cases of electrical oscillation are represented graphically in Fig. 468. Types of "oscillation" that result if the resistance is made equal to and greater than the critical damping value are shown in Fig. 469. Kelvin also found what determined the number of oscillations per second which the system would produce (thefrequency). He discovered it to depend primarily upon the inductance and capacitance in the circuit according to the relation (9) It is worthy of note that if equation (8) be solved for f, the same value results. But equation (8) expressed the condition that a circuit should "resonate" to an alternating voltage, the inductive and capacitative reactance being equal to each other. This will be recognized as another parallel between mechanical and electrical oscillation, for it is one of the best known phenomena of mechanics that resonance occurs when the natural frequency of the vibrating body is the same as that of the impressed vibration. One of Kelvin's comments is worthy of special notice, in view of subsequent occurrences. He expressed the opinion that an oscillatory discharge might be produced artificially from a Leyden phial or other conductor [sparking] across a very small space of air, and through a conductor of very large electrodynamic capacity 1 and small resistance. Should it be impossible on account 1 "Electrodynamic capacity" was the current term for inductance, the latter term not having yet come into use.

726

ELECTRICITY

of the too great rapidity of the successive flashes, for the unaided eye to distinguish them, Wheatstone's method of a revolving mirror might be employed and might show the spark as several points or short lines of light separated by dark intervals.

This experiment suggested by Kelvin was employed six years later by Fedderson 1 in a series of brilliant experiments. He identified in his revolving mirror the oscillatory nature of the spark and the circumstances under which oscillations degenerated into a single discharging surge; and he measured the frequency when oscillations were occurring. But it was soon recognized that resistance was not the only agency that acted to dissipate the energy of an oscillating circuit. Energy was also dissipated from such a circuit by radiation into surrounding space, much as a candle sends out energy in the form of light as well as by the heat it gives its surroundings. In 1864 J. Clerk Maxwell 2 developed the consequences of this concept in what was probably the most momentous mathematical discovery in the history of electrical science. This paper laid the foundation for the whole science of radio. Unfortunately it was too far ahead of its time to be comprehended. It was not until twenty years later that Heinrich Hertz, of Karlsruhe, Germany, produced and was able to receive at a distance, the electromagneti2 waves, which Maxwell had predicted. The series of engineering developments from that point on, beginning with Branley, Fleming, and Marconi and leading to radio as we know it now, is common knowledge. The series of events which Henry initiated in 1842 furnishes an excellent illustration of the interplay of experiment and mathematical theory in physics. Beginning with Henry's experiments, the major successive developments were alternately experimental and theoretical, one after the other, until the foundation was laid for perhaps the greatest engineering accomplishment of all time. Henry, Kelvin, Fedderson, Maxwell, Hertz; experiment, theory, experiment, theory, experiment. Not a single one of those stages could have been realized without the sequence that went before it. Even Henry's work was an outgrowth of his own earlier investigations and those of others. As a means of securing an insight in to the scientific method, one can do no better than to consider this example. The technological sequels, which form the subject matter of the next two chapters, are Ii ttle more than top foliage of this primary process, the taproot of our material civilization.

Problems on Chapter 47 1. A power line is distributing 1200 kilowatts of power at 2400 volts. What is the current if the power factor is .8? How much power is lost in the transmission line if it has a resistance of i ohm? 400 amps.; 80 kw. 1 2

Poggendorff's Annalen, 103, 69 (1859). Philosophical Transactions, 155, 419 (1864).

ALTERNATING CURRENTS AND OSCILLATING CIRCUITS

727

2. If the voltage were stepped up, by means of a transformer to 60,000 volts, and distributed at that voltage, what would be the current and loss of power? 16 amps.; 128 watts. 3. An inductance of .2 henry and a capacitance of 35 m.f. are connected in series. To what frequency will the combination resonate? 71. cycles per sec. 4. An inductance of .2 henry is connected in series with a variable capacitance. To what value should the latter be adjusted to cause the combination to resonate to a frequency of 100 cycles per second? 13 m.f. 5. To the combination of problem 4 a series resistance of 10 ohms is added and the whole connected to 100 volts at 100 cycles. \Vhat is the current when the capacitance is adjusted to 5, 12.67 and 20 m.f., in turn? .52, 10, and 2.1 amperes. 6. A coil has an inductance of L henrys. What is its reactance X in ohms for a frequency off cycles per second?

L 1. 1. 1. 0.03

5. 85. 7. An inductance of L henrys is connected in series with a resistance of R ohms. Calculate the impedance Z of the combination for a frequency of 60 cycles, and the effective current I which flows if the combination is connected to an alternating voltage of 115 volts 60 cycles.

L .03 .25 1.

5.

R 10 100 400 2000

x

f

25 60 500 60 60 60

150 370 3,100 0 1,900 32,000

z 15 140 550 2700

I 7.7 .82 .21 .043

8. An alternating current i may be expressed as a function of the maximum value I of the current, the frequency f, and the time t by the equation

i = I sin 21rft. If I= 10 amperes and f each ,,!0 of a sec. from t

720t

i

I o O

1

+5

2

= =

60 cycles/sec., find the value of i in amperes for 0 to t = -lo sec. 3

... + 10

4

5

.. + 5

6 7 ~ 9 10 11 12 0 - 5 ........................ 0

9. Plot the values of i obtained in problem 8 as ordinates against the corresponding values oft as abscissas. 10. An alternating voltage leads the current of problem 3 by a phase angle of~ (90°). The expression for it is e = E sin ( 21rft.

+ ~) .

60 cycles, find the values of e in vol ts for each t = -l0 sec.

1~0,

Given that E is 10 vol ts and f is 7

§ 0 of a second from t = 0 to

I+ ~-1 + ~-1 .. ~- ... ~- ... ~- .- ~.6 .. ~- .. -~ ... ~- .. ~- .. -1~- .+1~.6+ /~

11. Plot the values of e in problem 10 against the corresponding values of t on the same graph used in problem 9.

728

ELECTRICITY

12. Given a condenser of capacity C microfarads, find its reactance X in ohms for a frequency off cycles per second.

c 1. 1. 1. 0.03 0.03 0.03 0.03 0.001 0.001

5. 5. 100.

x

.f

6,400. 2,600. 320. 84,000. 10,500. 1,050. 10.5 260,000. 320. 520. 6.4 26.

25 60

500 60 500 5000

5· 105 60 5 · 105 60 5000 60

13. A condenser of capacity C microfarads is connected in series with a resistance R ohms and connected to 115 volts A.C. (effective), f cycles per second. Find the impedance Z in ohms for the combination and the effective current I in amperes that flows. c z R I f 100. 60 103. 1.1 100 530. ' 0.22 5. 60 100 0.044 1. 60 2600. 100 330. 0.34 1. 500 100 1.1 5000 100 105. 1. 0.11 0.03 100 1050. 5000 5· 105 1.1 0.03 100 100.5 0.001 5· 105 100 0.34 330. 14. A coil of inductance L henrys, a condenser of capacity C microfarads, and a resistance R ohms are connected in series. An effective voltage of 115 volts at 60 cycles is applied. Find the inductive reactance, XL, the capacitive reactance, Xe, the total reactance, X = XL - Xe, the impedance, Z, all in ohms, and the effective current, I. in amperes.

L 1. 1. 1.4 0.7

c 10 5 5 10

R 100 100 100 100

XL

Xe

x

z

380 380 527 264

270 530 530 265

110 - 150 3 1

150 210 100 100

I 0.77 0.55 1.1 1.1

15. An inductance of .2 henry, a capacitance of 35 microfarads, and a resistance of 10 ohms are connected in series. 100 volts A.C. is applied at a frequency off cycles per second. Calculate the impedance and current for each of the following frequencies, 0, 20, 40, 50, 55, 60, 70, 80, and 100. Plot the current as a function of the frequency. (Compare Fig. 467.) At exactly what frequency does resonance occur?

H

0 0

20 .5

40 1.9

50 3.2

55 6.

60 10.

65 6.4

70 4.

80 2.2

100 1.2

16. Repeat problem 15 for a resistance of 100 ohms, and plot the current against frequency on the same graph.

~I

0 0

20 .45

40 .76

50 .96

55 .99

60 1.

65 .99

70 .97

80 .91

100 .78"

ALTERNATING CURRENTS AND OSCILLATING CIRCUITS

729

17. A condenser of capacity 1 microfarad is charged to a potential difference V of 100 volts. I ts two terminals are then connected by a resistance of 1QG ohms. The voltage across the condenser is given as a function of the time by the equat

tion v = Ve- RC where e is the base of natural logarithms. Find the voltage v across the plates of the condenser for the following values of t in seconds. (See exponential table in appendix.) t

v

I 100o

.2

.4

.7

82

67

50

1. 37

2.

4.

13

2

18. Repeat problem 17 for R = 105 ohms.

: I

1~. 3/

.2

.3

.4

.5

13.

5.

1.8

.7

19. Plot values of v as ordinates against values oft as abscissas for problems 17 and 18. 20. An electrical appliance is found to use I amperes when connected to V volts. A wattmeter indicates that W watts of power are used. What is the power factor?

I 1 2 3 4

V 115 115 230 230

W 100 190 600 750

The greatest and noblest pleasure which men can have in this world is to discover new truths; and the next is to shake off old prejudices. FREDERICK THE GREAT, eighteenth century

P.F. 0.9 0.8 0.9 0.8

The Telegraph and Telephone The electric telegraph, if successful, would be an unmixed evil to society; would be used only by stock jobbers and speculators and the present Post Office is all the public utility requires. DR. BURBECK, of the Mechanics Institute, England, 1837

Essential Elements in Electrical Communication Underlying all the electrical arts of communication - and these include not only telegraphy and telephony but also telephotography, television and soundpictures - are a few basic principles. To appreciate those fundamentals is to possess a pass-key which opens to ready understanding the systems of communication which at first thought appear to have no essential in common. Whether the transmission medium is wire or wireless, whether the signals which convey the information are audible or visual, there is a unity to all the communication arts. However different their devices and contrivances may seem to be, all of them are merely means for performing one or another of six typical operations. Four of these operations are essential to any system of communication and the other two to all except the simplest systems. 1

four operations thus stated to be common to all devices for electrical communication are: (1) generation of some effect which can be transmitted; (2) modulation (or "molding") of the current which has been generated in accordance with the signals which are to be transmitted; (3) transmission to the point of destination; and (4) detection, the conversion of the modulated current into intelligible signals. The other two operations are: (5) amplification, which becomes necessary where great distance enfeebles the current; and (6) some process of selection, which is required when the transmitting medium is carrying more than one message at the same time. The first four operations are evident in the electric telegraph of Henry. The battery generated a current. His switch, which evolved into the telegraph key of Morse and of Wheatstone, modulated this current. Wires transmitted the current, and a bell - later a recording device and still later a telegraph ''sounder'' - detected the modulated current. It will be illuminating to describe other agencies of communication, notably the telephone and the radio, from the standpoint of the elements common to all electrical devices for conveying intelligence.

THE

1

John Mills, Bell Telephone Quarterly, 14, 13 (1935).

THE TELEGRAPH AND TELEPHONE

731

Obstacles to Early Development Electrical communication plays such a prominent part in modern life that it is hard to visualize the circumstances of a social order which felt no need for it and was in some ways hostile to it. It was, to be sure, scarcely surprising that even a well-informed ma:n should have said as early as 1837 that "the electric telegraph, if successful, would be an unmixed evil to society," but that it should have occurred to no American railroad official for seven years after Morse's patent was granted that the telegraph might be useful in dispatching trains is almost unbelievable. Perhaps it was natural for a southern Kentucky community to destroy a near-by telegraph line in 1849 on the basis that it robbed the air of its electricity, the rains are hendered, and ther' ain't been a good crop sence the wire was put up.

But that the United States War Department as late as 1885 should have declared officially that provision for electrical communication was not needed by the army, would pass the bounds of credibility were it not actually on record ( 114 :48 7). But, though the early lack of appreciation of the value of electrical communication was unedifying, some of the manifestations of its actual appreciation are even more so. It is a great misfortune that this inventive field has been the subject of more frequent and bitter dispute than any other. The three-cornered dispute between Morse, Wheatstone, and Henry set a precedent which was to be paralleled and even exceeded in the development of the telephone and radio. The victories in these disputes seem, regrettably, to have gone more often to the sides possessing the heaviest economic artillery than to those who could really present the best cases. While these disputes will not be of primary concern here, such evaluation of them as circumstances require will be made more from the evidence itself than from popular impressions of what that evidence indicates.

Early Telegraphy The three major improvements in telegraphy which gave the art its present place were submarine-cable telegraphy, multiplex telegraphy, and the printing telegraph, now called the teletype. The last two were almost entirely engineering problems, that is, the adaptation of well-known scientific principles to particular requirements. The first, however, was almost as much on the scientific as on the engineering frontier. It commanded the attention of Lord Kelvin and of Michael Faraday, both being physicists of the highest standing. A considerable portion of the early difficulties experienced with transatlantic telegraphy arose in fact from the disregard by "practical" engineers of the procedures specified by these men of science.

7J2

ELECTRICITY

There was, to be sure, no lack of purely engineering problems to be solved. The construction of rnbmarine cables was made possible in the first place by the invention of gutta-percha in 1847. This, with all its short-comings, was the only material then known that would perform satisfactorily the function of an insulating substance under such exacting conditions. The problems involved in laying such cables were also primarily within the engineering realm, and were of the first magnitude. Though a few relatively short cables had been laid during the fifties - across the English channel, between islands in the Mediterranean, and between coast cities in the United States - engineers of the sixties were for the most part without experience in cable manufacture or cable laying; and their acquirement of such experience in connection with the Atlantic cable was accompanied by several disastrous and very expensive failures. Four major unsuccessful attempts ate up millions of investors' dollars•and almost discredited the whole idea of a trans-oceanic cable. The engineering problems of laying the Atlantic cable, serious though they were, were rendered less troublesome than they might have been by the discovery in 1852-53 of a submarine plateau between Newfoundland and Ireland, the two natural termini of the main section of such a cable. Ordinarily the ocean-bottom has localities of rugged" scenery": rocky hills, deep gorges, swift submarine currents. But between Newfoundland and Ireland was found a smooth plain, such an ideal bed for a cable that it was immediately named Telegraph Plateau. It was deep enough to place the cable beyond the reach of anchors, icebergs, and drifts, yet shallow enough to render feasible the laying of cable. Microscopic shells, brought to the surface unbroken and unabraded, indicated the absence of destructive currents. Except for this purely fortuitous circumstance, the history of Atlantic cable-laying would have been even more checkered than it was. But besides the engineering problems, major scientific problems were involved in the operation of a cable as long as that across the Atlantic. The necessity for sufficiently delicate instruments spurred sensitive galvanometers into being long before they would otherwise have been designed, as has already been observed (page 656). But the main difficulty with the operation of the cable was the apparent sluggishness with which signals seemed to be transmitted. It was a difficulty which had been largely unforeseen and which has been ameliorated only within the last twenty years. The reason for this peculiarity was the fact that a cable was so long in comparison with the usual line, and its wires were so close together that a condenser of very large capacitance was produced. Transmission was slow because it was necessary to " fill " this condenser before a signal could emerge at the receiving end. Even upon emergence, the signal was so distorted by its experience that it was likely to be unrecognizable. Hence, even when conditions were at their best, the transmission of a message by

THE TELEGRAPH AND TELEPHONE

733

transatlantic cable was found to require much more time than by the short lines in use before then. No way of even ameliorating this handicap was devised from 1858 when it was first encountered until 1924. In that year Oliver Buckley of the Bell Telephone Laboratories introduced into the sheath of a new transatlantic cable a layer of tape of a newly devised alloy called permalloy. The peculiar magnetic properties of this alloy (high permeability, whence the prefix "perm") had the effect of increasing the inductance of the cable. The consequent lag in the phase angle of the current constituting the signal partly neutralized the lead produced by the high capacity of the cable (see page 722), and made it possible to send messages five or six times as rapidly. There are still serious strictures on the utility of long submarine cables, however. To this day they are totally unusable for telephonic purposes.

The Telephone In the telephone may be found a second illustration of the basic principles of electric communication. In this case a microphone or "transmitter" replaces the telegraph key as the modulator of the electric current. The current, instead of consisting of a succession of long and short signals according to a code, is molded to a wave-form similar to that of the sound waves incident on the transmitter. This fluctuating current is reconverted into sound by the receiver at the listening end of the line. The whole process is indicated in its simplest form in Fig. 470. The type of microphone commonly used in telephone practice effects changes in the resistance of a telephone circuit under the influence of incident sound waves, such that the resulting fluctuating current will be a passably faithful replica of the sound. The oscillation of a diaphragm alternately compresses and releases carbon granules contained in a connected pellet somewhat as idealized in Fig. 4 71. The receiver is essentially an electromagnet acting on a diaphragm. The incoming electric current, modulated according to the original sound pattern, sets the diaphragm in motion in a sequence of oscill. ations approximately similar to those executed by the diaphragm of the microphone at the other end of the. line. The corresponding sound waves, upon reaching

FIG.

470. A

SIMPLE TELEPHONE CIRCUIT

734

ELECTRICITY

the ear of the listener, complete the function which the telephone is designed to perform. The reader will recognize a certain parallel between the telephone receiver and the electric motor. Like the motor, the receiver responds with mechanical motion to an electric current, modulated for the purpose. This fact leads one to speculate whether the reciprocal relation between motor and generator applies to this case; that is, whether, upon imparting motion to the diaphragm of a receiver, a current will be generated in the winding of the electromagnet. FIG. 471. AcnoN OF CARBON-GRANULE The discovery that this was 'the MICROPHONE case was made in 1875 by Alex(Drawing reproduced by permission of John Mills, ander Graham Bell, and receivers Bell Telephone Laboratories.) thus used "in reverse" were at one time the only kind of transmitter generally known.

FIG. 472.

ADVERTISING POSTER OF ABOUT 1880, SHOWING REcEiVER USED ALSO AS TRANSMITTER

THE TELEGRAPH AND TELEPHONE

7J 5

The first well-authenticated electric telephone appears to have been devised in 1861 by Philipp Reis, a teacher of physics in Friedrichsdorf, Germany. His microphone consisted of a single loose contact between metals, and his receiver had no diaphragm other than the sounding-board to which the electromagnet was attached. But these circumstances merely reduced the efficiency of operation without affecting the principle. His telephone operated well enough to carry out Reis's main objective, which was to transmit speech. That he accomplished this has been amply confirmed by the unequivocal testimony of several reputable men of science who actually used Reis's original instruments, both under his direction and independently .1 Though there have been many other claimants to the distinction of having devised the first successful telephone mstrumen ts, Reis presents the clearest title of them all. The first words understood over a telephone in the United States were spoken by Alexander Graham Bell on March 10, 1876. The transmitter described and illustrated in his own patent, granted three days before, had been of the receiver type, mentioned above. But on the famous tenth of March, FIG. 473. THE TRANSMITTER UsEo BY BELL ON Bell used, not that transmitter, MARCH 10, 1876 but one which was a precursor of the microphone type. This latter transmitter had been very completely described and illustrated in a document submitted to the patent office nearly a month before, by a competitor of Bell named Elisha Gray, about which Bell subsequently acknowledged having received information prior to March 10. Bell sedulously refrained from mentioning his now famous exploit of 1876 for more than four years, and in the meantime his company developed the receiver type of transmitter exclusively. All this 1 Though Reis's success and even his intention to transmit speech have been vigorously denied and though the denial has been supported by court decisions in the United States, both the denial and the decisions seem to have been based on incomplete and ex parte testimony. The reader is referred to S. P. Thompson's Philipp Reis, Inventor of the Telephone (London, Spon, 1883) especially the Preface, pp. 36-38 and pp. 112 ff.

736

ELECTRICITY

indicates unmistakably what Bell thought during that time about where the credit belonged for the transmitter with which the first American telephone conversation was conducted. 1

Later Telephone Development The invention and perfection of the carbon-granule microphone spanned the eighties. It was first devised by an English clergyman named Hunnings, the principal improvements being by Thomas Edison and Anthony White. 2 Since that time neither the transmitter nor the receiver used in ordinary telephony has been modified in any respect except minor details. As applied to public address systems and to radio telephony, however, major modifications have been made and will be described in appropriate connections. The lack of improvements in modulation and detection of telephone messages during the last fifty years has been simply an outgrowth of the fact that by 1890 these two operations of telephony had developed to a point fifty or more years in advance of the intervening opIt is in this eration, transmission. operation that all the significant developments have occurred since 1890. They

1111~· ·---.-· .

~.

--

~

""""

FIG. 474. THE TRANSMITTER ILLUSTRATED IN

BELL'S

1876

PATENT

GRANTED

MARCH

7,

FIG. 475. THE TRANSMITTER ILLUSTRATED IN ELISHA GRAY'S CAVEAT OF FEB. 14, 1876

1 For a more extended account of the issue between Bell and Gray the reader is referred to The American Physics Teacher, 5, 243 (1937). 2 For a detailed history of the development of the microphone see The BeU Telephone Quarterly, JO, 164 (1931). This account, though well-documented, is unfortunately seriously in error at several important points.

THE TELEGRAPH AND TELEPHONE

737

have followed two main lines. The first is the improvement in central office equipment and organization, facilitating the control of ever-increasing numbers of calls with ever-decreasing delay and culminating in the machine-switching equipment - the prevalence of which is now indicated by the use of dial telephones in all large communities. The second is the extension of the distances over which telephone conversations could be conducted. The first has been primarily a matter of refinement of technological minutiae which need not concern us. The second, however, centers in two major scientific developments which are worthy of note: the development of the so-called loading coil and that of the electron-tube amplifier or repeater, as it is called in telephone practice.

The Extension of Telephonic Distance The circumstance that limited the range of early telephone conversation was the same as that which had slowed up communication over submarine cables, namely, the electrostatic capacitance of the lines. The difficulty was met in the case of the cables by retarding the speed at which telegraphic messages were clicked off. No such solution was possible for the telephone problem. This was partly because there was a definite lower limit to the speed of intelligible enunciation, and partly because the frequency of telephone signals (speech sounds) was in the range of hundreds or even a few thousands per second, whereas that of telegraph messages never exceeded eight or ten characters per second. The problem was solved in 1899 in somewhat the same way that a partial solution of the cable problem was effected many years later, namely, by neutralizing the effect of the capacitance of the cables by judicious introduction of inductances at intervals along the line. It was necessary that the distances between these inductances should not be greater than a certain fraction of the mean wave-length of the telephone messages, and that the magnitude of the inductances should be properly adjusted to produce a phase lag that should equal the phase lead which it was desired to neutralize. This difficult problem was solved by Michael I. Pupin (1858-1935), a professor in Columbia University, who in 1874 had landed in this country, a Serbian immigrant with just five cents in his pocket and not a word of English on his tongue. The effect of the cable's capacitance had been to cause voice-waves of different frequencies to travel with different speeds, thus distorting the emerging speech signals beyond the point of intelligibility. The "loading coils" which corrected this difficulty may now be seen placed in collections of iron boxes along transcontinental telephone cable lines at intervals of approximately a mile and a quarter. Pupin's solution for overland telephone cables was much more complete than Buckley's later solution of the submarine telegraph cable problem, notwithstanding the fact that the performance of telephone cables is immeasurably more

738

ELECTRICITY

exacting than that of telegraph cables. The impossibility of interpolating properly designed inductance coils at intervals along a submarine cable precludes the utilization of Pupin's discovery in that field. In 1902 public telephone service between New York and Chicago, made possible by the loading coil, was inaugurated with great ceremony. This was to mark the virtual maximum of telephone distance until the advent of the repeater in 1915, which made it possible to renew the energy of the voice currents at appropriate intervals along transcontinental lines. The repeater is simply a special form - for telephone use - of a device more commonly known as an amplifier. By means of it, weak electrical impulses can so control a strong local source of electric output as to mold it into the same forms. Thus a telephone current, which is weak to the point of utter inaudibility, upon passing through a repeater station emerges w,ith as great energy as at the source, or even greater. This amplification may be repeated as many times along a line as is necessary to reach a required distance. The heart of the amplifier is a device which in its usual forms, presents somewhat the appearance of a peculiarly-shaped electric light globe. It is variously known as a thermionic vacuum tube, a three-electrode vacuum tube, or a triode. It originated in an observation by Edison in 1883 in connection with his study of electric illuminants. To investigate the cause of an asymmetrical blackening of the inside surfaces of his early lamps, he had sealed an additional electrode into the side of one of them. He observed that a small current could be made to flow across the evacuated space between the incandescent filament and this second electrode if the latter were given a positive polarity, but not otherwise. Since no possible use could be imagined for this peculiar phenomenon, Edison merely recorded it, but gave it no further attention. In 1889 an English scientist showed that the effect was due to a flow of electrons, released from the filament by its high temperature, to the second electrode when attracted by a positive charge. This accounted for the absence of a contrary flow when the polarity of the second electrode was reversed. A German investigator promptly utilized the phenomenon to "rectify" alternating current. A common modern example of this process is to be found in the '' Tungar '' rectifier used to charge storage batteries from A.C. power lines. The next two stages in the development of the vacuum tubeand, except for minor improvements, _ _ the final stages - were made by two FIG. 476. A RECTIFIED ALTERNATING early workers in the field of wireless CURRENT WAVE FoRM telegraphy. It occurred in 1904 to

f\

(\

THE TELEGRAPH AND TELEPHONE

739

James A. Fleming, an Englishman who was one of Marconi's associates, that the rectifying action of the vacuum tube might solve the most troublesome problem of detection of wireless waves. His subsequent development of what is known in England to this day as the "Fleming valve" gave the vacuum tube its first real start to fame. In 1906, Lee Deforest, an American radio experimenter and promoter, not satisfied with the performance of the Edison-Fleming tube, modified it by introducing the element which, above all others, rendered the vacuum tube adaptable to the purposes it is now serving. He tried the experiment of placing between the filament and the second electrode (now called the plate) a tiny grid with bars of fine wire, through which he hoped to secure an extra measure of control over the cloud of electrons occupying that space when the tube was in operation. This step proved to be a stroke of genius, the results of which must have been a surprise even to Deforest himself. There has probably never been another single invention that has influenced technology so profoundly. The fact that it facilitated long-distance telephony is one of its lesser outgrowths. Most of the modern attributes of FIG. 477. THE INTERIOR ARRANGEMENT OF A THREEradio communication are traceable to the ad- ELECTRODE TUBE dition of a third electrode to the vacuum tube, and the broader fields of electrical engineering are finding new uses for the three-electrode tube almost every day. It is indeed quite impossible to overestimate the ultimate importance of Deforest's contribution to vacuum tube design. The principle of the action of the third electrode is simple. Thrust into the electron cloud which always exists between the plate and the incandescent filament, it is in a position to exercise, through its electrical potential, an intimate control over the motions of these electrons. A certain negative potential will stop the flow of electrons from the filament to the positively charged plate; a smaller negative potential will permit a limited flow; a positive potential,1 if not too great, will cause the flow to be greater than it would have been in the absence of a grid. Thus the flow of electrons from filament to plate or, in the usual terminology, the plate current is intimately controlled by the grid potential. With the proper attention to design and operation, changes in plate current may be made proportional to changes in grid potential. Small changes in potential may thus, through the agency of a three-electrode tube, control a local source of electrical en1

As usually arranged, a triode seldom utilizes a positive potential on its grid.

740

ELECTRICITY

FIG. 478. A SECTION OF THE FIRST "COAXIAL CABLE" CONSISTING OF A PAIR OF WIRES EACH IN THE Axis OF A COPPER CYLINDER (Courtesy of Bell Telephone Company.)

ergy m such a way as to produce in it relatively large but proportional surges. This is the principle of operation of the tube used as an amplifi~r. If one such amplifier is insufficient, its output may constitute the input of a second tube. Though there are practical limits to the number of such stages of amplification, set by a sort of electrical analogy to the economic "law of diminishing returns," six or seven stages are common, giving amplification ratios up to many millionfold. This is the action of the telephone "repeater," more familiar examples being the action of public address systems and ordinary radio receiving sets.

Refinements of Telephony Under certain circumstances several telephone messages may be sent simultaneously over the same wire. In the case of the so-called "coaxial cable" between New York and Philadelphia, as many as two hundred and forty two-way messages may be transmitted at one time. In such cases the technique is similar to that of the generation and reception of radio messages. The telephone transmitters each modulate an alternating current of different frequency, instead of modulating direct currents as in the case of the ordinary telephone. The frequencies of the alternating currents are so great as to be beyond the audible range; hence, the ear hears only the modulated waves and is entirely unaware of the carrier wave. Each of the receiving instruments is tuned to the frequency of the corresponding transmitter. The signals travel along a wire, instead of through space. The sending of photographs by wire, which has in recent years become one of the routine features of news dispatch, differs only in detail from sending telegrams or conversations by wire. The chief difference is that the electrical current is modulated by a light beam instead of by a sound wave at the sending end and is reconverted into a correspondingly fluctuating beam by the receiver. A microscopic view of a picture sent by wire shows it to consist of lines either varying in density or varying in width, to produce the required effects. These lines are produced by a process of

THE TELEGRAPH AND TELEPHONE

74 I

FIG. 479. A PHOTOGRAPH AND A MAGNIFIED SECTION SENT OVER A WIRE IN LINES OF VARIABLE DENSITY

scanning at the sending end. A tiny pencil of light moves across the transparency to be transmitted, in successive, closely spaced parallel lines. The fluctuations in intensity of the portion of the beam that penetrates the transparency go over the line in the form of fluctuations of current. These actuate a light valve at the receiving end, controlling the intensity of a pencil of light to duplicate the variations at the transmitting station. The details of how these steps are effected are not of present concern. The important point is to realize that in principle the operation of sending pictures by wire is the same as that of sending conversations by wire. The process of (1) generation, (3) transmission, (5) amplification, and (6) selection (see page 730) are identical in the two cases. Only (2) modulation and (4) detection are different because the original and final forms FIG. 480. A PORTION' OF A TELEPHOTOGRAPH are light patterns instead of PRODUCED BY LINES OF VARIABLE WIDTH

•

742

ELECTRICITY

FIG.

481. A TELEPHOTOGRAPHIC

INSTALLATION, TRANSMITTING MACHINE IN THE FOREGROUND, WITH PICTURE IN PLACE (Courtesy of Bell Telephone Company.)

sound patterns. The process of broadcast television differs from that of telephotography somewhat as moving picture photography differs from ordinary still photography. Instead of there being nearly twenty minutes to send a picture, as in telephotography, a televised picture must be completed in about one-sixteenth of a second. During that brief interval the scanning, transmission, and restoration at the receiving end must be completed, for to avoid flicker, sixteen such pictures must succeed each other

Telephone circuit

Amplifier - - - - - - Motors controlled to run at same speed-----Sending apparatus

Receiving apparatus

FIG. 482. A TELEPHOTOGRAPHIC SYSTEM IN 0uTLINE (Drawing reproduced by permission of Bell Telephone Company.)

THE TELEGRAPH AND TELEPHONE

743

every second. The basic process is the same in both cases, however, though the mechanism involved in so enormous a speeding-up is necessarily different.

The Photoelectric Cell Playing a central role in telephotography as well as in television is a device which will merit special attention by way of concluding this chapter. It is known as the photoelectric cell. It is the principal agent through which electric currents may be modulated by light patterns. The pencil of light which penetrates the transparency at the sending end of the line strikes a photoelectric cell which, with the aid of its accessories, acts to convert the fluctuating intensities of light into corresponding fluctuations of current. It depends on a phenomenon, first observed by Hallwachs in 1888, that under appropriate conditions electrons are liberated from metal surfaces by light, especially by ultra-violet light. The shortness of the interval between the incidence of the light and the release of electrons was astonishing, being only a few billionths of a second. Amplification of the small potentials thus created made it possible to use the photoelectric cell to effect modulation of an electric current by light. In recent years, synthetic surfaces have been constructed which are enormously more effective than metals. 1 A different type of cell, which generates currents instead of merely emitting electrons when exposed to light, is becoming familiar to photographers as the working nucleus of the "exposure meter." The current generated in such cells is caused to actuate a delicate but rugged galvanometer, so that the whole compact assembly can be used to make a rapid determination of the intensity of light. These cells are sometimes termed "photovoltaic," a very descriptive name, to distinguish them from the older type termed ''photoelectric.''

FIG. 483. A COMMON TYPE OF PHOTOELECTRIC CELL (Courtesy of Weston Electrical Instrument Company.) 1

Notably that devised by M.

FIG. 484. A

COMMON TYPE OF EXPOSURE METER (Courtesy of Weston Electrical Instrument Company.)

J. Kelley of the Bell Telephone Laboratories. See

Laboratories Record, October, 1933.

Bell

Radio Communication Here is unfolded to us a new and astonishing world - one which it is hard to conceive should contain no possibilities of transmitting and receiving intelligence ... Here then is revealed the bewildering possibility of telegraphy without wires, posts, cables, or any of our present costly appliances. CROOKES, 1892 I

The Versatility of Radio Radio is the miracle of the ages. Aladdin's Lamp, the Magic Carpet, the Seven League Boots of fable and every vision that mankind has ever entertained, since the world began, of laying hold upon the attributes of the Almighty, pale into insignificance beside the accomplished fact of radio. By its magic the human voice may be projected around the earth in less time than it takes to pronounce the word " radio." The story of wireless broadcasting has ramifications as ancient as civilization because since time began men have struggled with the very riddle to which this latest triumph of human ingenuity brings solution. Blind gropings of generations of experimenters have contributed to progress along the pathway to the pinnacle from which ... science suddenly glimpsed the secret of radio broadcasting. (12 :3).

is playing an ever-increasing part in human life on land, on sea, and in the air. It makes possible a great deal more than the mere communication of news, entertainment, and messages from place to place. It plays an important part in navigation. The radio beacon and the radio compass permit the pilot and mariner to locate positions in fog and storm. The radio printer delivers his weather maps daily. Without radio, travel by air would not be, as it is fast becoming, one of the safest methods of transportation. Radio instruments indicate the absolute altitude of the plane, guide the airliner from port to port, and facilitate landing, making "blind" flying possible. The pictures in our daily newspapers are transmitted by wireless. Even the geophysicist does prospecting by means of radio instruments. A detailed explanation of the construction and operation of the instruments performing any one of these functions would involve technical developments beyond the scope of this work. The fundamental principles basic to the science of radio communication are, however, within the grasp of the beginning student. RADIO

RADIO COMMUNICATION

745

In the opening paragraphs of the chapter on the telegraph and telephone six typical operations involved in various methods of communication were enumerated. Each of these six operations plays an important part in radio communication. They are important enough to warrant restatement here. These six operations are: (1) generation of some effect which can be transmitted, this effect being called in radio the carrier wave, or carrier; (2) modulation of the carrier by the signal to be transmitted; (3) transmission to the receiver; (4) selection of the modulated carrier at the receiver, this process being commonly called tuning; (5) detection, or conversion of the modulated carrier into intelligible signals; and (6) amplification of weak electric currents.

The Transmission of Sound over a Light Beam Light beams have occasionally been used as carriers of sound. Though such an experiment is more of a "stunt" than a scientific or utilitarian measure, it furnishes an apt illustration of some of the principles of radio. The process is shown in Fig. 485. The carrier is a beam of light generated in S, a special light source operating on principles similar to the familiar neon lamp. In fact a small neon lamp may be used. This type of lamp has its light produced by an electrical discharge in a rarefied gas and differs from the ordinary incandescent, or filament type, lamp in that the intensity of the light emitted can be made to change very rapidly. The battery B1 furnishes the voltage to operate the light source S. As long as the current through this lamp remains constant the intensity of the light emitted remains constant. If, however, fluctuating current, produced when sound strikes the microphone M, is sent through the primary Ballast resistance

Amplifier Speaker

Amplifier

M FIG. 485.

THE TRANSMISSION OF SOUND OVER A LIGHT BEAM

746

ELECTRICITY

of the transformer T, the fluctuating voltage produced in the secondary of this transformer is superimposed on the voltage already applied by the battery B1 to the lamp S. This causes the current through the lamp, and hence the light radiated from it, to fluctuate in intensity in accordance with the sound vibrations striking the microphone. The light is then said to be modulated by the sound current originating in the microphone M. The light radiated from the source falls on the lens L1 which converts it into a nearly parallel beam. The apparatus described thus far may be called the transmitter. The modulated beam of light is directed at the lens L2, which focuses it on the photoelectric cell P. The photoelectric cell, as explained in the preceding chapter, has the property of conducting electricity in proportion to the amount of light incident upon it. Thus, when the light beam is c