Part 24

At the meeting of the British Association in 1876 Sir W. Thomson gave the following account of the performance of this instrument at the Philadelphia Exhibition:—“In the Canadian department” (for Professor Bell was not at the time an American citizen) “I heard ‘To be or not to be—there’s the rub,’ through the electric wire; but, scorning monosyllables, the electric articulation rose to higher flights, and gave me passages taken at random from the New York newspapers:—‘S. S. Cox has arrived’ (I failed to make out the ‘S. S. Cox’), ‘the City of New York,’ ‘Senator Morton,’ ‘the Senate has resolved to print a thousand extra copies,’ ‘the Americans in London have resolved to celebrate the coming Fourth of July.’ All this my own ears heard spoken to me with unmistakable distinctness by the thin circular disc armature of just such another little electro-magnet as this which I hold in my hand. The words were shouted with a clear and loud voice by my colleague judge, Professor Watson, at the far end of the line, holding his mouth close to a stretched membrane, carrying a piece of soft iron, which was thus made to perform in the neighbourhood of an electro-magnet, in circuit with the line, motions proportional to the sonorific motions of the air. This, the greatest by far of all the marvels of the electric telegraph, is due to a young countryman of our own, Mr. Graham Bell, of Edinburgh, and Montreal, and Boston, now about to become a naturalized citizen of the United States. Who can but admire the hardihood of invention which devised such very slight means to realize the mathematical conception that, if electricity is to convey all the delicacies of quality which distinguish articulate speech, the strength of its current must vary continuously, and as nearly as may be in simple proportion to the velocity of a particle of air engaged in constituting the sound?”

Since these words were spoken by one of the highest authorities in matters telegraphic, Professor Bell has introduced some important modifications in his apparatus. He now employs, not an electro-magnet, but a permanent magnet. That is to say, instead of using at each station such a bar of soft iron as is shown in Fig. 6, which becomes a magnet while the electric current is passing through the coil surrounding it, he uses at each station a bar of iron permanently magnetized (or preferably a powerful magnet made of several horse-shoe bars—that is, a compound magnet), surrounded similarly by coils of wire. No battery is needed. Instead of a current through the coils magnetizing the iron, the iron already magnetized causes a current to traverse the coils whenever it acts, or rather whenever its action changes. If an armature were placed across its ends or poles, at the moment when it drew that armature to the poles by virtue of its magnetic power, a current would traverse the coils; but afterwards, so long as the armature remained there, there would be no current. If an armature placed near the poles were shifted rapidly in front of the poles, currents would traverse the coils, or be induced, their intensity depending on the strength of the magnet, the length of the coil, and the rapidity and range of the motions. In front of the poles of the magnet is a diaphragm of very flexible iron (or else some other flexible material bearing a small piece of iron on the surface nearest the poles). A mouthpiece to converge the sound upon this diaphragm substantially completes the apparatus at each station. Professor Bell thus describes the operation of the instrument:—“The motion of steel or iron in front of the poles of a magnet creates a current of electricity in coils surrounding the poles of the magnet, and the duration of this current of electricity coincides with the duration of the motion of the steel or iron moved or vibrated in the proximity of the magnet. When the human voice causes the diaphragm to vibrate, electrical undulations are induced in the coils around the magnets precisely similar to the undulations of the air produced by the voice. The coils are connected with the line wire, and the undulations induced in them travel through the wire, and, passing through the coils of another instrument of similar construction at the other end of the line, are again resolved into air undulations by the diaphragm of this (other) instrument.”

So perfectly are the sound undulations repeated—though the instrument has not yet assumed its final form—that not only has the lightest whisper uttered at one end of a line of 140 miles been distinctly heard at the other, but the speaker can be distinguished by his voice when he is known to the listener. So far as can be seen, there is every room to believe that before long Professor Bell’s grand invention will be perfected to such a degree that words uttered on the American side of the Atlantic will be heard distinctly after traversing 2000 miles under the Atlantic, at the European end of the submarine cable—so that Sir W. Thomson at Valentia could tell by the voice whether Graham Bell, or Cyrus Field, or his late colleague Professor Watson, were speaking to him from Newfoundland. Yet a single wave of those which toss in millions on the Atlantic, rolling in on the Irish strand, would utterly drown the voices thus made audible after passing beneath two thousand miles of ocean.

Here surely is the greatest of telegraphic achievements. Of all the marvels of telegraphy—and they are many—none are equal to, none seem even comparable with, this one. Strange truly is the history of the progress of research which has culminated in this noble triumph, wonderful the thought that from the study of the convulsive twitchings of a dead frog by Galvani, and of the quivering of delicately poised magnetic needles by Ampère, should gradually have arisen through successive developments a system of communication so perfect and so wonderful as telegraphy has already become, and promising yet greater marvels in the future.

The last paragraph had barely been written when news arrived of another form of telephone, surpassing Gray’s and La Cour’s in some respects as a conveyor of musical tones, but as yet unable to speak like Bell’s. It is the invention of Mr. Edison, an American electrician. He calls it the motograph. He discovered about six years ago the curious property on which the construction of the instrument depends. If a piece of paper moistened with certain chemical solutions is laid upon a metallic plate connected with the positive pole of a galvanic battery, and a platinum wire connected with the negative pole is dragged over the moistened paper, the wire slides over the paper like smooth iron over ice—the usual friction disappearing so long as the current is passing from the wire to the plate through the paper. At the receiving station of Mr. Edison’s motograph there is a resonating box, from one face of which extends a spring bearing a platinum point, which is pressed by the spring upon a tape of chemically prepared paper. This tape is steadily unwound, drawing by its friction the platinum point, and with it the face of the resonator, outwards. This slight strain on the face of the resonator continues so long as no current passes from the platinum point to the metallic drum over which the moistened tape is rolling. But so soon as a current passes, the friction immediately ceases, and the face of the resonator resumes its normal position. If then at the transmitting station there is a membrane or a very fine diaphragm (as in Reuss’s or Bell’s arrangement) which is set vibrating by a note of any given tone, the current, as in those arrangements, is transmitted and stopped at intervals corresponding to the tone, and the face of the resonating box is freed and pulled at the same intervals. Hence, it speaks the corresponding tone. The instrument appears to have the advantage over Gray’s in range. In telegraphic communication Gray’s telephone is limited to about one octave. Edison’s extends from the deepest bass notes to the highest notes of the human voice, which, when magnets are employed, are almost inaudible. But Edison’s motograph has yet to learn to speak.

Other telegraphic marvels might well find a place here. I might speak of the wonders of submarine telegraphy, and of the marvellous delicacy of the arrangements by which messages by the Atlantic Cable are read, and not only read, but made to record themselves. I might dwell, again, on the ingenious printing telegraph of Mr. Hughes, which sets up its own types, inks them, and prints them, or on the still more elaborate plan of the Chevalier Bonelli “for converting the telegraph stations into so many type-setting workshops.” But space would altogether fail me to deal properly with these and kindred marvels. There is, however, one application of telegraphy, especially interesting to the astronomer, about which I must say a few words: I mean, the employment of electricity as a regulator of time. Here again it is the principle of the system, rather than details of construction, which I propose to describe. Suppose we have a clock not only of excellent construction, but under astronomical surveillance, so that when it is a second or so in error it is set right again by the stars. Let the pendulum of this clock beat seconds; and at each beat let a galvanic current be made and broken. This may be done in many ways—thus the pendulum may at each swing tilt up a very light metallic hammer, which forms part of the circuit when down; or the end of the pendulum may be covered with some non-conducting substance which comes at each swing between two metallic springs in very light contact, separating them and so breaking circuit; or in many other ways the circuit may be broken. When the circuit is made, let the current travel along a wire which passes through a number of stations near or remote, traversing at each the coils of a temporary magnet. Then, at each swing of the pendulum of the regulating clock, each magnet is magnetized and demagnetized. Thus each, once in a second, draws to itself, and then releases its armature, which is thereupon pulled back by a spring. Let the armature, when drawn to the magnet, move a lever by which one tooth of a wheel is carried forward. Then the wheel is turned at the rate of one tooth per second. This wheel communicates motion to others in the usual way. In fact, we have at each station a clock driven, _not_ by a weight or spring and with a pendulum which allows one tooth of an escapement wheel to pass at each swing, but by the distant regulating clock which turns a driving wheel at the rate of one tooth per second, that is, one tooth for each swing of the regulating clock’s pendulum. Each clock, then, keeps perfect time with the regulating clock. In astronomy, where it is often of the utmost importance to secure perfect synchronism of observation, or the power of noting the exact difference of time between observations made at distant stations, not only can the same clock thus keep time for two observers hundreds of miles apart, but each observer can record by the same arrangement the moment of the occurrence of some phenomenon. For if a tape be unwound automatically, as in the Morse instrument, it is easy so to arrange matters that every second’s beat of the pendulum records itself by a dot or short line on the tape, and that the observer can with a touch make (or break) contact at the instant of observation, and so a mark be made properly placed between two seconds’ marks—thus giving the precise time when the observation was made. Such applications, however, though exceedingly interesting to astronomers, are not among those in which the general public take chief interest. There was one occasion, however, when astronomical time-relations were connected in the most interesting manner with one of the greatest of all the marvels of telegraphy: I mean, when the _Great Eastern_ in mid-ocean was supplied regularly with Greenwich time, and this so perfectly (and therefore with such perfect indication of her place in the Atlantic), that when it was calculated from the time-signals that the buoy left in open ocean to mark the place of the cut cable had been reached, and the captain was coming on deck with several officers to look for it, the buoy announced its presence by thumping the side of the great ship.

_THE PHONOGRAPH, OR VOICE-RECORDER._

In the preceding essay I have described the wonderful instrument called the telephone, which has recently become as widely known in this country as in America, the country of its first development. I propose now briefly to describe another instrument—the phonograph—which, though not a telegraphic instrument, is related in some degree to the telephone. In passing, I may remark that some, who as telegraphic specialists might be expected to know better, have described the phonograph as a telegraphic invention. A writer in the _Telegraphic Journal_, for instance, who had mistaken for mine a paper on the phonograph in one of our daily newspapers, denounced me (as the supposed author of that paper) for speaking of the possibility of crystallizing sound by means of this instrument; and then went on to speak of the mistake I (that is, said author) had made in leaving my own proper subject of study to speak of telegraphic instruments and to expatiate on the powers of electricity. In reality the phonograph has no relation to telegraphy whatever, and its powers do not in the slightest degree depend on electricity. If the case had been otherwise, it may be questioned whether the student of astronomy, or of any other department of science, should be considered incompetent of necessity to describe a telegraphic instrument, or to discuss the principles of telegraphic or electrical science. What should unquestionably be left to the specialist, is the description of the practical effect of details of instrumental construction, and the like—for only he who is in the habit of using special instruments or classes of instrument can be expected to be competent adequately to discuss such matters.

Although, however, the phonograph is not an instrument depending, like the telephone, on the action of electricity (in some form or other), yet it is related closely enough to the telephone to make the mistake of the _Telegraphic_ journalist a natural one. At least, the mistake would be natural enough for any one but a telegraphic specialist; the more so that Mr. Edison is a telegraphist, and that he has effected several important and interesting inventions in telegraphic and electrical science. For instance, in the previous article, pp. 270, 271, I had occasion to describe at some length the principles of his “Motograph.” I spoke of it as “another form of telephone, surpassing Gray’s and La Cour’s in some respects as a conveyer of musical tones, but as yet unable to speak like Bell’s ... in telegraphic communication.” I proceeded: “Gray’s telephone is limited to about one octave. Edison’s extends from the deepest bass notes to the highest notes of the human voice, which, when magnets are employed, are almost inaudible; but it has yet to learn to speak.”

The phonograph is an instrument which _has_ learned to speak, though it does not speak at a distance like the telephone or the motograph. Yet there seems no special reason why it should not combine both qualities—the power of repeating messages at considerable intervals of time after they were originally spoken, and the power of transmitting them to great distances.

I have said that the phonograph is an instrument closely related to the telephone. If we consider this feature of the instrument attentively, we shall be led to the clearer recognition of the acoustical principles on which its properties depend, and also of the nature of some of the interesting acoustical problems on which light seems likely to be thrown by means of experiments with this instrument.

In the telephone a stretched membrane, or a diaphragm of very flexible iron, vibrates when words are uttered in its neighbourhood. When a stretched membrane is used, with a small piece of iron at the centre, this small piece of iron, as swayed by the vibrations of the membrane, causes electrical undulations to be induced in the coils round the poles of a magnet placed in front of the membrane. These undulations travel along the wire and pass through the coils of another instrument of similar construction at the other end of the wire, where, accordingly, a stretched membrane vibrates precisely as the first had done. The vibrations of this membrane excite atmospheric vibrations identical in character with those which fell upon the first membrane when the words were uttered in its neighbourhood; and therefore the same words appear to be uttered in the neighbourhood of the second membrane, however far it may be from the transmitting membrane, so only that the electrical undulations are effectually transmitted from the sending to the receiving instrument.

I have here described what happened in the case of that earlier form of the telephone in which a stretched membrane of some such substance as goldbeater’s skin was employed, at the centre of which only was placed a small piece of iron. For in its bearing on the subject of the phonograph, this particular form of telephonic diaphragm is more suggestive than the later form in which very flexible iron was employed. We see that the vibrations of a small piece of iron at the centre of a membrane are competent to reproduce all the peculiarities of the atmospheric waves which fall upon the membrane when words are uttered in its neighbourhood. This must be regarded, I conceive, as a remarkable acoustical discovery. Most students of acoustics would have surmised that to reproduce the motions merely of the central parts of a stretched diaphragm would be altogether insufficient for the reproduction of the complicated series of sound-waves corresponding to the utterance of words. I apprehend that if the problem had originally been suggested simply as an acoustical one, the idea entertained would have been this—that though the motions of a diaphragm receiving vocal sound-waves _might_ be generated artificially in such sort as to produce the same vocal sounds, yet this could only be done by first determining what particular points of the diaphragm were centres of motion, so to speak, and then adopting some mechanical arrangements for giving to small portions of the membrane at these points the necessary oscillating motions. It would not, I think, have been supposed that motions communicated to the centre of the diaphragm would suffice to make the whole diaphragm vibrate properly in all its different parts.

Let us briefly consider what was before known about the vibrations of plates, discs, and diaphragms, when particular tones were sounded in their neighbourhood; and also what was known respecting the requirements for vocal sounds and speech as distinguished from simple tones. I need hardly say that I propose only to consider these points in a general, not in a special, manner.

We must first carefully draw a distinction between the vibrations of a plate or disc which is itself the source of sound, and those vibrations which are excited in a plate or disc by sound-waves otherwise originated. If a disc or plate of given size be set in vibration by a blow or other impulse it will give forth a special sound, according to the place where it is struck, or it will give forth combinations of the several tones which it is capable of emitting. On the other hand, experiment shows that a diaphragm like that used in the telephone—not only the electric telephone, but such common telephones as have been sold of late in large quantities in toy shops, etc.—will respond to any sounds which are properly directed towards it, not merely reproducing sounds of different tones, but all the peculiarities which characterize vocal sounds. In the former case, the size of a disc and the conditions under which it is struck determine the nature of its vibrations, and the air responds to the vibrations thus excited; in the latter, the air is set moving in vibrations of a special kind by the sounds or words uttered, and the disc or diaphragm responds to these vibrations. Nevertheless, though it is important that this distinction be recognized, we can still learn, from the behaviour of discs and plates set in vibration by a blow or other impulse, the laws according to which the actual motions of the various parts of a vibrating disc or plate take place. We owe to Chladni the invention of a method for rendering visible the nature of such motions.

Certain electrical experiments of Lichtenberg suggested to Chladni the idea of scattering fine sand over the plate or disc whose motions he wished to examine. If a horizontal plate covered with fine sand is set in vibration, those parts which move upwards and downwards scatter the sand from their neighbourhood, while on those points which undergo no change of position the sand will remain. Such points are called _nodes_; and rows of such points are called _nodal lines_, which may be either straight or curved, according to circumstances.

If a square plate of glass is held by a suitable clamp at its centre, and the middle point of a side is touched while a bow is drawn across the edge near a corner, the sand is seen to gather in the form of a cross dividing the square into four equal squares—like a cross of St George. If the finger touches a corner, and the bow is drawn across the middle of a side, the sand forms a cross dividing the square along its diagonals—like a cross of St Andrew. Touching two points equidistant from two corners, and drawing the bow along the middle of the opposite edge, we get the diagonal cross and also certain curved lines of sand systematically placed in each of the four quarters into which the diagonals divide the square. We also have, in this case, a far shriller note from the vibrating plate. And so, by various changes in the position of the points clamped by the finger and of the part of the edge along which the bow is drawn, we can obtain innumerable varieties of nodal lines and curves along which the sand gathers upon the surface of the vibrating plate.

When we take a circular plate of glass, clamped at the middle, and touching one part of its edge with the finger, draw the bow across a point of the edge half a quadrant from the finger, we see the sand arrange itself along two diameters intersecting at right angles. If the bow is drawn at a point one-third a quadrant from the finger-clamped point, we get a six-pointed star. If the bow is drawn at a point a fourth of a quadrant from the finger-clamped point, we get an eight-pointed star. And so we can get the sand to arrange itself into a star of any even number of points; that is, we can get a star of four, six, eight, ten, twelve, etc., points, but not of three, five, seven, etc.

In these cases the centre of the plate or disc has been fixed. If, instead, the plate or disc be fixed by a clip at the edge, or clamped elsewhere than at the centre, we find the sand arranging itself into other forms, in which the centre may or may not appear; that is, the centre may or may not be nodal, according to circumstances.