David Edward Hughes(1831-1900)

THE ENGINEER

MAY 17,1878

PROFESSOR D. E. HUGHES’S TELEPHONE, MICROPHONE, AND THERMOPILE.

THE various branches of physical science are so inter-connected that any important discovery in one branch is frequently the forerunner of a number of discoveries not only in that particular direction, but in cognate branches. Such has been the case as regards the researches and discoveries of Reiss, Gray, Edison, and Bell. The human mind, having its energies once turned in a certain direction, does not remain satisfied until it meets with an impenetrable blank- not, however, to remain a blank for ever, but only until other discoveries give the means to overcome difficulties. Nature has long ago learned how to bottle-up the sunshine and hide it in the bowels of the earth till toiling man claims the treasure, and utilizes anew the long, long pent-up force. Man is now emulating nature, and bottles up sound to reproduce at his own convenience. Her results, however, can only be obtained by acting in accordance with Nature’s laws. A new discovery, then, means the discovery of a new law or a new application of an old law. Physicists have long agreed that heat and sound are modes of motion. Heat, however, cannot be observed unless the matter in motion impinges on a material object whose molecules have not the same motion. Similarly sound is only heard when the motion is capable of actuating the organs of the ear. The introduction of the telephone has given an impetus to the study of vocal sounds, and the researches of Prof. Fleeming (sic) Jenkin and others lead us to hope, as Mr. A. J. Ellis says, “that we have at last got an instrument which will enable us to solve the elementary problems of phonetics that have hitherto almost baffled us, although it is not suited, as yet, to fix those delicacies of utterance which were my own special object of investigation.”

As we proceed with this article we shall see that the discovery of Professor Hughes has given Mr. Ellis a far more delicate instrument than that of Professor Bell, and thus the investigation into the laws of acoustics can be carried further. Professor Hughes reasoned that as the electrical condition of substances is affected by heat, and in some cases at least by light, and that heat and light being undulating motions, therefore the electrical condition should be affected by sound, sound being, as in the other cases, the production of motion. To investigate this he “made a rough-and-ready telephone, with a small bar magnet 4in. long, half the coil of an electro-magnet, and a square piece of ferrotype iron, 3in. square, clamped rigidly in front of one pole of the magnet between two pieces of board.” Professor Hughes is justified in calling attention to what he terms rough-and-ready apparatus. Old match-boxes, money-boxes, empty cigar boxes, and pieces of material that most philosophers would despise, have in his hands been fashioned and manipulated to give astounding results.

His battery consists of three small tumblers- as shown in Fig.1- containing a coil of copper at the bottom, on which is placed some sulphate of copper, and then filled up with clay well moistened with water. On the top is a small plate of zinc. This battery, we are informed, has been in use for three months, and works as well as ever. The internal resistance is of course considerable, as is required. The battery is connected with the telephone above-mentioned, and with the materials upon which the experiment is made.

The first investigations of Professor Hughes will best be explained in his own language:- “I introduced into the circuit at S (vide Fig.1) a strained conductor- a stretched wire, listening attentively with the telephone to detect any change that might occur when the wire was spoken to or set into transverse vibrations by being plucked aside. Gradually, till the wire broke, the strain was varied, but no effect whatever was remarked except at the moment when the wire broke. The effect was but momentary, but invariably at the moment of breaking a peculiar ‘rush’ or sound was heard. I then sought to imitate the condition of the wire at the moment of rupture by replacing the broken ends and pressing them together with a constant and varying force by the application of weights. It was found that if the broken ends rested upon one another with a slight pressure of not more than one ounce to the square inch on the joins, sounds were distinctly reproduced, although the effects were very imperfect.” The next act was to find means to produce the sound other than by the breaking of the wire, and to find a substance or combination of substances which when influenced by sound waves would transmit the sound to the telephone. The Professor found that if he filled a small glass tube with a mixture of tin and zinc, known as “white bronze”, plugging the ends with pieces of carbon, and connecting the whole as in Fig.2 with a galvanometer in circuit, on slightly pressing the carbons, and thus compressing the metallic particles, he obtained a deflection in one direction, whilst on exerting a tensile strain on the glass, and thus slightly expanding the space occupied by the metallic particles, he obtained a deflection in the other direction. On compressing the metallic particles they are brought into closer and better contact, and the total resistance of the circuit is decreased, but on the contrary, when the particles are farther apart the electrical contact is not so good and the resistance is increased. Here then was the basis on which to build.

The materials to be acted upon must be such that the molecules would answer to the impingement of the sound wave, and thus make an alteration in the resistance of the circuit. One of the transmitters devised by Professor Hughes is shown in Fig.3, and is described by him as follows: “The tube transmitter consists of an exterior glass tube two inches long, and one quarter of an inch in diameter. In it are four separate pieces of willow charcoal, each one a quarter of an inch long, and two terminals of the same material. The terminals are fastened in the tube and connected externally with the line, and internally with the four loose pieces. In this case A is made to press on B, C, D, E, and F, until the resistance offered to the electrical current is about one-third that of the line on which it is employed. It may be attached to a resonant board by the ends A or F. If the result were simply due to vibrations we should have A and B making greater contact at a different time from E and F, and consequent interference. If it were a simple shaking or moving of B, C, D, E, and F it would produce no change, as, if B pressed more strongly on C, it would be less on A, and also if the tube were attached by the center we should have no effect; but if the effect be due to a swelling or enlargement of B, C, D, E, and F, it would make no difference where it is attached to the resonant board, as is actually the case. Again, reduce the pressure of A upon B, &c., until they are not in contact, and no trace of current can be perceived by shaking the tube. The instant the sonorous vibrations pass in the tube there is electrical contact to a remarkable degree, which could only have taken place by the molecules enlarging their sphere under the influence of the sonorous vibrations.”

As we have seen, sound is heard on the breaking of the wire, and also on putting the ends together; but proceeding onwards, Professor Hughes found that it was not necessary to join the ends of the wire. So long as good contact was made, little difference occurred in the results. The ends of the wires were connected to two French nails laid parallel on a board, and contact made with a coin, a third nail, a piece of watch-spring, a piece of chain; in fact, it can be made with any clean metallic surface – Fig.4.

Fine metallic filings introduced at the points of contact greatly added to the perfection of the results. “At this point,” says Professor Hughes, “articulate speech became clearly and distinctly reproduced, together with its timbre, and I found that all that now remained was to discover the best material and form to give to this arrangement its maximum effect. I tried all forms of pressure and modes of contact; a lever, a spring, pressure in a glass tube sealed up while under the influence of strain, so as to maintain the pressure constant. All gave similar and invariable results, but the results varied with the material used. All metals, however, could be made to produce identical results, provided the division of the metal was small enough, and that the material does not oxidize by contact with the air filtering through the mass.” The best material seems to be metallized carbon. A piece of willow carbon heated to a white heat and plunged under mercury becomes filled with mercury particles in a minute state of subdivision, and what was previously almost a non-conductor has now gained conducting properties. Carbon similarly metallized with iron gives good results. Any of these preparations provided with wires for insertion in the circuit is termed the transmitter. The receiving instrument is the telephone. As is well known, the great objection to the telephone has been the weakness of the sound given out. The Hughes transmitter commences a new era in telephonic use. The theory put forward by Professor Hughes is very ingenious. “It is quite evident that the effects are due to a difference of pressure at the different points of contact, and that they are dependent for the perfection of action upon the number of these points of contact. Moreover, they are not dependent upon any apparent difference in the bodies in contact, but the same body, in a state of minute subdivision, is equally effective. Electrical resistance is a function of the mass of the conductor, but sonorous conduction is a function of the molecules of matter. How is it, therefore, that a sonorous wave can so affect the mass of a conductor as to influence its electrical resistance.” If we assume a line of molecules at the point of contact of the minute masses of conducting matter in their normal condition to be arranged as in the first group in Fig.5, they will appear under compression as in the central group, and under dilation as in the last.

Hence, in the one case, the resistance is less because of the closer proximity of a larger number of points, and greater in the other by reason of the increased distance between the points. If we look upon a molecule as a minute substance constantly in motion, with an influence over a spherical portion of space, vast in comparison to itself, and that the space over which this influence is exerted is lessened or increased by the action of the sonorous vibrations, we can easily see how the increased and decreased resistance would occur without imagining change in the shape of the molecule.

In the ordinary telephone the sound wave is made to impinge upon a diaphragm which causes magnetic changes in a permanent magnet, and electrical changes in the coil surrounding the magnet, but Professor Hughes dispenses with the diaphragm. You speak at the French nails, or the tubes prepared as above described, and the sonorous waves directly effect the required result. The following experiments can easily be made. Take a prepared tube and fasten it on the top of an empty money box with one end taken out, speak or sing into the box, and the sound is reproduced clear and distinct on the receiving telephone – Bell’s. Nay, you need not speak into the box, put the box to your ear, or your forehead, or your foot if you like, and talk away – the sound is transmitted all the same. Speak to the coin as in Fig.4, and the sound traverses the circuit to the ear at the distant telephone. But these extraordinary results rapidly pale before others more wonderful still. The apparatus as shown in Fig.6, and which has been termed a microphone, consists solely of a piece of metallized carbon, balanced on a pivot and connected with one pole of the battery through the telephone; this piece of carbon can rest on another piece of metallized carbon connected with the other pole of the battery.

Sixpence and sixteen minutes’ attention would enable any one to make this wonderful microphone. Speak to it a yard away and the sound is conveyed with distinctness; touch the wood upon which it rests with the softest camel’s hair brush and you have the sound of sawing wood at the receiving end; touch with the holder of the brush and you hear the harsh grating sound as if carpenters were sharpening their saws; take prisoner a common house fly, incarcerate him in a match-box surrounded with gauze, and place the prisoner in his prison-house near the microphone, and with ear at the telephone you hear him tramp as he walks. This instrument is destined to prove of the utmost importance in the hands of the physician. The noises in the chest, the beatings of the heart, will all be laid bare with a distinctiveness never before known and hardly ever conceived. Another form of this instrument perhaps more delicate still is given in Fig.7.

This, indeed, seems to approach the confines of perfection. You need not trouble to be within a foot or a yard of the instrument. It is a verbatim reporter. A speaker might have a score or so of such reporters before him connected electrically with the principal towns in the kingdom, and his words would be repeated distinctly. The ordinary voice if too near sounds harsh and disagreeable. There is an old saying that “Walls have ears,” and it is now so far true that double doors and walls cannot keep secret any conversation held within. A singular fact about the whole of these experiments is that the people at both ends of the line may be talking or singing simultaneously, and the conversation or song is transmitted without interference to the other end, but you do not hear your own speech or song at your own telephone. Here then we can have a duplex system without any special apparatus, and probably it will be found that not only two, but many times two, messages are capable of being transmitted at the same time.

A discovery, not made till after Professor Hughes’s paper was read before the Royal Society, points out another field of usefulness for this instrument. We all know what excellent service the thermo-electric pile has done in the hands of Professor Tyndall and other investigators of heat, but we think we may safely describe another kind of thermo-pile as sensitive and far less complicated, less difficult to construct, and less expensive than that compounded of zinc and antimony in the usual fashion. Instead of the glass tube as described in Fig.2, Professor Hughes was experimenting with a quill tube, Fig.8, and found that the instrument was exceedingly sensitive to heat.

On the approach of a warm hand the galvanometer needle swings violently in one direction; on cooling the tube it swings the other. We have seen Prof. Hughes place a small French clock near the apparatus, and the motion of the clock generated heat sufficient to cause the swinging of the needle, and on allowing the small bell of the clock to strike, the needle swung violently as far as it would go. We have said enough to show the delicacy of all these instruments, but we must warn readers that these results have been obtained not with materials manufactured in the best possible way, but with materials taken from whatever source happened to be at hand, and put together in the roughest manner. Prof. Hughes has declined to patent his inventions, but gives them, with whatever value they possess, to the world. The world is deeply indebted to the discoverer for this noble act, and although many will not realize the sacrifice he has made, those who trouble themselves to think of the manifold uses to which the apparatus may at once be put, and of the numberless benefits which may accrue from its introduction, will deal out their praises with no niggard hand.

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