CHAPTER XXVI

NEWTON, FARADAY AND BELL AT WORK

Newton, the supreme generalizer . . . Faraday, the master of experiment . . . Bell, the inventor of the telephone, transmits speech by a beam of light.

Having now taken a rapid general view of observation and experiment, of the faculty of sound theorizing, let us enter the presence of two great masters of research and invention, beginning with a man who united the loftiest powers as a mathematician, a physicist, and a generalizer.

How Newton Discovered the Law of Gravitation.

How Sir Isaac Newton discovered the law of gravitation is thus told in his Life by Sir David Brewster:--“It was either in 1665 or 1666 that Newton’s mind was first directed to the subject of gravity. He appears to have left Cambridge some time before August 8, 1665, when the college was dismissed on account of the plague, and it was, therefore, in the autumn of that year, and not in that of 1666, that the apple is said to have fallen from the tree at Woolsthorpe, and suggested to Newton the idea of gravity. When sitting alone in the garden, and speculating on the power of gravity, it occurred to him that, as the same power by which the apple fell to the ground was not sensibly diminished at the greatest distance from the centre of the earth to which we can reach, neither at the summits of the loftiest spires, nor on the tops of the highest mountains, it might extend to the moon and retain her in her orbit, in the same manner as it bends into a curve the path of a stone or a cannon ball, when projected in a straight line from the surface of the earth. If the moon was thus kept in her orbit by gravitation, or, in other words, its attraction, it was equally probable, he thought, that the planets were kept in their orbits by gravitating towards the sun. Kepler had discovered the great law of the planetary motions, that the squares of their periodic times were as the cubes of their distances from the sun, and hence Newton drew the important conclusion that the force of gravity, or attraction, by which the planets were retained in their orbits, varied as the square of their distances from the sun. Knowing the force of gravity at the earth’s surface, he was, therefore, led to compare it with the force exhibited in the actual motion of the moon, in a circular orbit; but having assumed that the distance of the moon from the earth was equal to sixty of the earth’s semi-diameters, he found that the force by which the moon was drawn from its rectilinear path in a second of time was only 13.9 feet, whereas at the surface of the earth it was 16.1 in a second. This great discrepancy between his theory and what he then considered to be the fact, induced him to abandon the subject, and pursue other studies with which he had been occupied.

“It does not distinctly appear at what time Newton became acquainted with the more accurate measurement of the earth, executed by Picard in 1670, and was thus led to resume his investigations. Picard’s method of measuring his degree, and the precise result which he obtained, were communicated to the Royal Society, January 11, 1672, and the results of his observations and calculations were published in the Philosophical Transactions for 1675. But whatever was the time when Newton became acquainted with Picard’s measurement, it seems to be quite certain that he did not resume his former thoughts concerning the moon until 1684. Pemberton tells us, that ‘some years after he laid aside’ his former thoughts, a letter from Dr. Hooke put him on inquiring what was the real figure in which a body, let fall from any high place, descends, taking the motion of the earth round its axis into consideration, and that this gave occasion to his resuming his former thoughts concerning the moon, and determining, from Picard’s recent measures, that ‘the moon appeared to be kept in her orbit purely by the power of gravity.’ But though Hooke’s letter of 1679 was the occasion of Newton’s resuming his inquiries, it does not fix the time when he employed the measures of Picard. In a letter from Newton to Hailey, in 1686, he tells him that Hooke’s letters in 1679 were the cause of his ‘finding the method of determining the figures, which, when I had tried in the ellipsis, I threw the calculations by, being upon other studies; and so it rested for about five years, till, upon your request, I sought for the papers.’ Hence Mr. Rigaud considers it clear, that the figures here alluded to were the paths of bodies acted upon by a central force, and that the same occasion induced him to resume his former thoughts concerning the moon, and to avail himself of Picard’s measures to correct his calculations. It was, therefore, in 1684, that Newton discovered that the moon’s deflection in a minute was sixteen feet, the same as that of bodies at the earth’s surface. As his calculations drew to a close, he is said to have been so agitated that he was obliged to desire a friend to finish them.”

Michael Faraday’s Method of Working.

With no mathematics beyond simple arithmetic, Michael Faraday displayed powers of experiment and generalization so extraordinary that in these respects he stands at the same height as Newton himself. In the life of Michael Faraday, by Dr. J. H. Gladstone, we are given his account of the great physicist’s method of working:--

“The habit of Faraday was to think out carefully beforehand the subject on which he was working, and to plan his mode of attack. Then, if he saw that some new piece of apparatus was needed, he would describe it fully to the instrument maker with a drawing, and it rarely happened that there was any need of alteration in executing the order. If, however, the means of experiment existed already, he would give Anderson, his assistant, a written list of the things he would require, at least a day before--for Anderson was not to be hurried. When all was ready, he would descend into the laboratory, give a quick glance round to see that all was right, take an apron from the drawer, and rub his hands together as he looked at the preparations made for his work. There must be no tool on the table but such as he required. As he began his face would be exceedingly grave, and during the progress of an experiment all must be exceedingly quiet; but if it was proceeding according to his wish, he would commence to hum a tune, and sometimes to rock himself sideways, balancing alternately on either foot. Then, too, he would often talk to his assistant about the result he was expecting. He would put away each tool in its own place as soon as done with, or at any rate as soon as the day’s work was over, and he would not unnecessarily take a thing away from its place. No bottle was allowed to remain without its proper stopper; no open glass might stand for a night without a paper cover; no rubbish was to be left on the floor; bad smells were to be avoided if possible; and machinery in motion was not to be permitted to grate. In working, also, he was very careful not to employ more force than was wanted to produce the effect. When his experiments were finished and put away, he would leave the laboratory, and think further about them upstairs.

“It was through this lifelong series of experiments that Faraday won his knowledge and mastered the forces of nature. The rare ingenuity of his mind was ably seconded by his manipulative skill, while the quickness of his perceptions was equalled by the calm rapidity of his movements. He had indeed a passion for experimenting. This peeps out in the preface to the second edition of his ‘Chemical Manipulation,’ where he writes, ‘Being intended especially as a book of instruction, no attempts were made to render it pleasing, otherwise than by rendering it effectual; for I concluded that, if the work taught clearly what it was intended to inculcate, the high interest always belonging to a well-made or successful experiment would be sufficient to give it all the requisite charms, and more than enough to make it valuable in the eyes of those for whom it was designed.’

“He could scarcely pass a gold leaf electrometer without causing the leaves to diverge by a sudden flick from his silk handkerchief. I recollect, too, his meeting me at the entrance to the lecture theatre at Jermyn Street, when Lyon Playfair was giving the first, or one of the first lectures ever delivered in the building. ‘Let us go up here,’ said he, leading me far away from the central table. I asked him why he chose such an out-of-the-way place. ‘Oh,’ he replied, ‘we shall be able here to find out what are the acoustic qualities of the room.’

“The simplicity of the means with which he made his experiments was often astonishing, and was indeed one of the manifestations of his genius. A good instance is thus narrated by Sir Frederick Arrow:--‘When the electric light was first permanently exhibited at Dungeness, on 6th June, 1862, a committee of the Elder Brethren, of which I was one, accompanied Faraday to observe it. Before we left Dover, Faraday showed me a little common paper box and said, “I must take care of this; it’s my special photometer,”--and then, opening it, produced a lady’s ordinary black shawl pin (jet, or imitation, perhaps)--and then holding it a little way off the candle, showed me the image very distinct; and then, putting it a little further off, placed another candle near it, and the relative distance was shown by the size of the image.’

“In lecturing to the young he delighted to show how easily apparatus might be extemporized. Thus, in order to construct an electrical machine, he once inverted a four-legged stool to serve for the stand, and took a white glass bottle for the cylinder. A cork was fastened into the mouth of this bottle, and a bung was fastened with sealing wax to the other end: into the cork was inserted a handle for rotating the bottle, and in the centre of the bung was a wooden pivot on which it turned: while with some stout wire he made crutches on two of the legs of the stool for the axles of this glass cylinder to work upon. The silk rubber he held in his hand. A japanned tea cannister resting on a glass tumbler formed the conductor, and the collector was the head of a toasting fork. With this apparently rough apparatus he exhibited all the rudimentary experiments in electricity to a large audience.”

Faraday’s Orderliness and Imagination.

Faraday, in addition to the rarest ability in experiment, had an orderliness of mind which gave the utmost effectiveness to his work in every department. His successor, Professor John Tyndall, says:--

“Faraday’s sense of order ran like a luminous beam through all the transactions of his life. The most entangled and complicated matters fell into harmony in his hands. His mode of keeping accounts excited the admiration of the managing board of the Royal Institution. And his science was similarly ordered. In his Experimental Researches he numbered every paragraph, and welded their various parts together by incessant reference. His private notes of the Experimental Researches which are happily preserved, are similarly numbered; their last paragraph bears the number 16,041. His working qualities, moreover, showed the tenacity of the Teuton. His nature was impulsive, but there was a force behind the impulse which did not permit it to retreat. If in his warm moments he formed a resolution, in his cool ones he made that resolution good. Thus his fire was that of a solid combustible, not that of a gas, which blazes suddenly, and dies as suddenly away.”

Faraday had exalted powers of imagination and as he gazed at the curves in which iron-filings disposed themselves when tapped on a card held above a magnet, he saw similar “lines of force” surrounding every attracting mass of whatever kind. Other observers had confined their attention to what takes place, or is supposed to take place, in a conductor; he closely scanned what took place around a conductor. He was thus addressed in a letter from that remarkable physicist, Professor James Clerk Maxwell of Cambridge:--

“As far as I know you are the first person in whom the idea of bodies acting at a distance by throwing the surrounding medium into a state of constraint has arisen, as a principle to be actually believed in. We have had streams of hooks and eyes flying around magnets, and even pictures of them so beset; but nothing is clearer than your description of all sources of force keeping up a state of energy in all that surrounds them, which state by its increase or diminution measures the work done by any change in the system. You seem to see the lines of force curving round obstacles and driving plump at conductors, and swerving toward certain directions in crystals, and carrying with them everywhere the same amount of attractive power, spread wider or denser as the lines widen or contract. You have seen that the great mystery is, not how like bodies repel and unlike attract, but how like bodies attract by gravitation. But if you can get over that difficulty either by making gravity the residual of the two electricities or by simply admitting it, then your lines of force can ‘weave a web across the sky’ and lead the stars in their courses without any necessarily immediate connection with the objects of their attraction. . . .”

How Light Becomes a Bearer of Speech.

Michael Faraday, as we have seen, by researches of consummate ability laid the foundation of modern electrical science and art. In that field there is to-day no inventor more illustrious than Professor Alexander Graham Bell, the creator of the telephone, that simplest and most important of electrical devices.[34] Not content with obliging a wire to carry speech in electric waves, Professor Bell has impressed beams of light into the same service. The successive steps by which he arrived at the photophone are of extraordinary interest. His story as given in the proceedings of the American Association for the Advancement of Science, 1880, is here somewhat condensed:--

[34] Professor Bell’s narrative of how he invented the telephone is given in “Invention and Discovery,” one of the six volumes of “Little Masterpieces of Science,” Doubleday, Page & Co., New York. In “Flame, Electricity and the Camera” by the present writer, published by the same firm, is a chapter describing the telephone in its later developments. This chapter was revised by the late Professor Alexander Melville Bell, father of the inventor.

“In bringing before you some discoveries by Mr. Sumner Tainter and myself, which have resulted in the production and reproduction of sound by means of light, let me sketch the state of knowledge which formed the starting point of our experiments. I shall first describe selenium, and the uses of it devised by previous experimenters; our researches have so widened the class of substances sensitive, like selenium, to light-vibrations that this sensitiveness seems to be a property of all matter. We have found this property in gold, silver, platinum, iron, steel, brass, copper, zinc, lead, antimony, german-silver, ivory, celluloid, gutta percha, hard and soft rubber, paper, parchment, wood, mica, and silvered glass. At first carbon and microscope glass seemed insensitive; later experiments proved them to be no exceptions to the rule.

“We find that when a vibratory beam of light falls upon these substances they emit sounds, the pitch of which depends upon the frequency of the vibratory change in the light. We also find that when we control the form or character of the light-vibrations, we control the quality of the sound, and obtain all varieties of articulate speech. We can thus speak from station to station wherever we can project a beam of light. Selenium, indispensable in the apparatus, was discovered by Berzelius in 1817. It is a metalloid resembling tellurium; they differ, however, in electrical properties; tellurium is a good conductor, selenium in its usual forms is a non-conductor. Knox, in 1837, discovered that selenium is a conductor when fused; in 1851, Hittorf showed that it conducts when in one of its allotropic forms. When selenium is rapidly cooled from a fused condition it is a non-conductor. In this vitreous form it is dark brown, almost black by reflected light, having an exceedingly brilliant surface; in thin films it is transparent, and appears of a beautiful ruby red by transmitted light. When selenium is cooled from fusion with extreme slowness, it presents an entirely different appearance, being of a dull lead color, and having throughout a granular or crystalline structure and looking like a metal. It is now opaque even in very thin films. It was this kind of selenium that Hittorf found to be a conductor of electricity at ordinary temperatures. He also noticed that its resistance to the passage of electricity diminished continuously by heating up to the point of fusion; and that the resistance suddenly increased as the solid passed to liquidity. It was early discovered that exposure to sunlight hastens the change of selenium from one allotropic form to another; an observation of significance in the light of recent discoveries.

The Cardinal Discovery.

“Mr. Willoughby Smith, an engineer engaged in the laying of submarine cables, had devised a system of testing and signalling during their submersion. For this system, in 1872, it occurred to him that he might employ crystalline selenium, on account of its high resistance, at the shore end of a cable. On experiment the selenium was found to have all the resistance required; some of the bars displayed a resistance of 1400 megohms, as much as would be offered by a telegraph wire long enough to reach from the earth to the sun. But this resistance was found to be extremely variable; the reason was disclosed when Mr. May, an assistant, observed that the resistance of selenium is less in light than in darkness. This discovery created widespread interest throughout the world. Among the investigators who at once turned their attention to the subject was Professor W. G. Adams of King’s College, London, who proved that the action on selenium is chiefly due to the luminous rays of the spectrum, the ultra-red and ultra-violet rays having little or no effect. Dr. Werner Siemens, the eminent German physicist, produced a variety of selenium fifteen times more conductive in sunlight than in darkness. This extraordinary sensitiveness was brought about by heating for some hours at a temperature of 210° C., followed by extremely slow cooling.

The Telephone Brought in.

“Observations concerning the effect of light upon the conductivity of selenium had employed the galvanometer solely; it occurred to me that the telephone, from its extreme sensitiveness, might be substituted with advantage. On consideration I saw that the experiments could not be conducted in the ordinary way with continuous light, for a good reason: the law of audibility of the telephone is precisely analogous to the law of electrical induction. No effect is produced during the passage of a continuous and steady current. It is only at the moment of change from a stronger to a weaker state, or, vice versa, that any audible effect is produced; this effect is exactly proportional to the amount of variation in the current. It was, therefore, evident that the telephone could only respond to the effect produced in selenium at the moment of change from light towards darkness, or _vice versa_, and that it would be advisable to intermit the light with great rapidity so as to produce a succession of changes in the conductivity of the selenium corresponding in frequency to musical vibrations within the limits of the sense of hearing. For I had often noticed that currents of electricity, so feeble as hardly to produce any audible effects from a telephone when the circuit was simply opened and closed, caused very perceptible musical sounds when the circuit was rapidly interrupted; and that the higher the pitch of the sound the more audible was its effect. I was much struck by the idea of producing sound in this way by the action of light. Accordingly I proposed to pass a bright light through one of the orifices in a perforated screen consisting of a circular disk with holes near its circumference. Upon rapidly rotating the disk an intermittent beam of light would fall on the selenium, and from a connected telephone a musical tone would be produced, its pitch depending upon the rapidity with which the disk spun round.

Variations of Light Necessary.

“Upon further consideration I saw that the effect could not only be produced at the extreme distance at which selenium would normally respond to the action of a luminous body, but that this distance could be indefinitely increased by using a parallel beam of light, so that we might telephone from one place to another with no conducting wire between the transmitter and the receiver. To reduce this idea to practice it was necessary to devise an apparatus to be operated by the voice of a speaker, by which variations could be produced in a parallel beam of light, corresponding to variations in the air produced by the voice. I proposed, therefore, to pass light through two plates perforated by many small orifices. One of these plates was to be fixed, the other was to be attached to the centre of a diaphragm actuated by the voice. In its vibrations the diaphragm would cause the movable plate to slide to and fro over the surface of the fixed plate, by turns enlarging and contracting the free orifices for the passage of light. The parallel beam emerging from this apparatus could be received at some distant place on a lens focussing it upon a sensitive piece of selenium placed in a local circuit, with a telephone and a galvanic battery. The variations in the light produced by a speaker’s voice should cause corresponding variations in the electrical resistance of the selenium at the distant place, and the telephone in circuit with the selenium should reproduce audibly the tones and articulations of the speaker’s voice. It is greatly due to the genius and perseverance of my friend, Mr. Sumner Tainter, that the problem thus entered upon has been successfully solved.

Special Treatment of the Selenium.

“The first point to which we devoted our attention was reducing the resistance of crystalline selenium within manageable limits. The resistance of selenium cells, employed by former experimenters, was counted in millions of ohms; there is no record of a cell measuring less than 250,000 ohms in the dark. We have succeeded in producing cells measuring only 300 ohms in the dark and 150 in the light. Our predecessors all seemed to have used platinum for the conducting part of their cells, excepting Werner Siemens, who found that iron and copper would do. We have discovered that brass, although chemically acted upon by selenium, forms an excellent material; indeed, we are inclined to believe that the chemical action between brass and selenium has contributed to the lowness in resistance of our cells, an intimate union taking place between the two substances. In brass we have constructed many cells of diverse forms. One of them (two are described by Professor Bell), is cylindrical so that it may be used with a concave reflector instead of with a lens. It is composed of many metallic disks separated by mica disks slightly smaller in diameter. The spaces between the brass disks over the mica are filled with selenium, and the alternate brass disks are metallically connected. The selenium is applied to the cell duly heated: next comes annealing. To effect this an oven is inserted in a pot of linseed oil standing upon glass supports in another similar pot of linseed oil. The whole is then heated to about 214° C., and kept there for twenty-four hours, then allowed to cool down during forty to sixty hours until the temperature of ordinary air is reached.

A Perfected Transmitter.

“We have devised more than fifty forms of photophonic transmitters. In one of them (several others are described by Professor Bell), a beam of light passes through a lens of variable focus formed of two sheets of thin glass or mica containing between them a transparent liquid or gas. When vocal vibrations are communicated to this gas or liquid, they cause a vibratory change in the convexity of the glass surfaces with a corresponding change in the intensity of the light as it falls upon the selenium. We have found the simplest apparatus to consist in a plane mirror of flexible material, such as silvered mica or microscope glass, against the back of which the speaker’s voice is directed.

“A large number of trials of this apparatus have been made with the transmitting and receiving instruments so far apart that sounds could not be heard directly through the air. In a recent experiment Mr. Tainter operated the transmitting instrument, placed on the top of the Franklin School House in Washington, D. C.; the receiver being arranged in a window of my laboratory, at a distance of 213 metres. Upon placing the telephone to my ear, I heard distinctly from the illuminated receiver: ‘Mr. Bell, if you hear what I say, come to the window and wave your hat.’

“We have found that articulate speech can be reproduced by the oxyhydrogen light, and even by a beam from a kerosene lamp. The loudest effects follow upon interrupting the light by means of a perforated disk swiftly rotated. Because this apparatus is noiseless it allows a close approach of the receiver while not interfering with its message.

“We have endeavored to ascertain the nature of the rays which affect selenium, placing in the path of an intermittent beam various absorbing substances. In these experiments Professor Cross has rendered us aid. When a solution of alum, or bisulphide of carbon, is employed, there is but slight reduction in loudness, but a solution of iodine in bisulphide of carbon cuts off most of the audible effect. Even an opaque sheet of hard rubber is less obstructive.

Experiments Without a Telephone.

“It is a well known fact that the molecular disturbance produced in a mass of iron by the magnetizing influence of an intermittent electrical current can be observed as sound by placing the ear in close contact with the iron. It occurred to us that the molecular disturbance produced in crystalline selenium by the action of an intermittent beam of light should be audible in a similar manner with no telephone or battery. Many experiments were made to verify this theory; at first without definite results. The behavior of the hard rubber just mentioned suggested listening to it also. This was tried with an extraordinary result. I held the sheet in close contact with my ear while a beam of intermittent light was focussed upon it through a lens. A distinct musical note was immediately heard. Other substances, as enumerated at the outset of my address, were now successively tried in the form of thin disks, in every case with success. On the whole, we feel warranted in announcing as our conclusion that sounds can be produced by the action of a variable light from substances of all kinds in the form of thin diaphragms. The reason why thin diaphragms are more effective than masses appears to be that the molecular disturbance produced by light is chiefly a surface action, and that the vibration has to be transmitted through the mass of the substance in order to affect the ear. We have led air, directly in contact with an illuminated surface, to the ear by throwing the luminous beam upon the interior of a tube. We have thus heard from interrupted sunlight very perceptible musical tones through tubes of ordinary vulcanized rubber, of brass, and of wood. These were all the materials at hand in tubular form, and we have had no opportunity since of extending the observations to other substances. A musical tone can be heard by throwing the intermittent beam of light into the ear itself. This experiment was at first unsuccessful on account of the position in which the ear was held.”