Popular Science Monthly/Volume 14/December 1878/Edison's Telephonic and Acoustic Inventions
By GEORGE B. PRESCOTT.
SOME of the discoveries and inventions of Mr, Edison, that were made during His researches which culminated in the invention of the carbon-telephone, have already been published. We now propose to present a more complete description of the important forms of telephone upon which he then experimented, and to describe his more recent acoustic inventions.
The carbon-telephone is only one of many contrivances for reproducing articulate speech at a distance through the aid of electricity, but, owing to its clear and truthful articulation, its simplicity of construction, and the far greater volume of sound which it creates, is probably destined to be the most extensively used. Other instruments of Mr. Edison's invention, however, are not far behind it, and may by improvement be made equally effective.
As a rule, Mr. Edison has succeeded better with those telephones which produce a variation in the resistance of the circuit than with those which depend for their action upon a variation of the electro-motive force or static charge. An instrument very similar to the carbon-transmitting telephone is shown in Fig. 1, the essential difference being that the carbon is replaced by bibulous paper moistened with water. This semi-conductor, like the carbon, changes its resistance under the influence of varying pressure. The paper is kept moist by capillary action, a strip being used, one end of which dips into a reservoir of water.
In Fig. 2 is shown a form of the carbon-transmitting telephone, requiring no adjustment whatever. It operates well, notwithstanding the simplicity of its construction. A plate of metal rests on the bottom of a hollow vessel. On this is placed a block of prepared carbon, upon which a second and light plate is laid.
The weight of the upper plate affords an initial pressure which is varied by speaking into the mouth of the vessel.
The carbon-block may be replaced by a disk of cloth, the pores of which have been filled with pulverized black-lead. By this treatment the cloth becomes slightly conductive. The instrument thus modified is shown in Fig. 3.
In Fig. 4 the pulverized plumbago, P, is floated upon mercury, M, and is compressed between the surface of the mercury and a metallic block fastened to the centre of the diaphragm.
Still another form of the Edison transmitter is shown in Fig. 5. The carbon, C, rests upon the diaphragm, which in this instrument is an horizontal plate forming the top of a vocalizing chamber, the mouth-piece being at the side.
Three fine cords attach the carbon to the framework of the diaphragm, and prevent it from being displaced when the diaphragm is vibrating. In appearance this instrument resembles the Reiss telephone, and in principle it would be much the same were it not that, in vibrating, the carbon never actually leaves the plate upon which it rests, but simply for an instant releases its pressure. It is evident that the resistance of the circuit depends upon the electric connection between the carbon and the diaphragm, and that this connection depends upon the pressure of the carbon, which is constantly changing when the diaphragm is in vibration.
Another form, acting on much the same principle, is illustrated by Fig. 6. It is called the inertia-telephone, though it is hardly certain that its action is to be attributed solely to inertia. The carbon, C, is placed between two metallic plates, one of which is fastened to the diaphragm, and the other is held by a screw, bearing in a framework attached to the diaphragm by insulating supports.
When vibrating, the whole system moves, instead of the plate P alone, as in the ordinary carbon-transmitter. Mr. Edison's explanation of its mode of action is, that the degree of pressure with which the carbon rests against the plates is varied during the vibration. Thus, after a movement toward the right, the diaphragm suddenly stops and the carbon presses, in virtue of its inertia, on the plate P.
An advantage which the magneto-telephone has over the carbon-telephone is that its diaphragm does not touch anything, and can therefore vibrate with perfect freedom. On the other hand, the diaphragm of the carbon-telephone presses with considerable force upon the carbon. In the form shown in Fig. 7 this difficulty is not encountered.
The diaphragm carries an armature. A, of soft iron, which confronts but does not touch the magnet B. A and B are opposite poles of the same magnet, being connected at P and polarized by a local circuit. The magnet B presses upon the carbon at C, the pressure being regulated by the screw S. The attraction between A and B varies with the distance between them. When, in vibrating, A moves toward B, the attraction rapidly increases, and B lessens its pressure upon C. During a motion in the opposite direction the attraction diminishes, and B, drawn by the screw S, increases its pressure upon C.
A similar contrivance is illustrated in Fig. 8. The diaphragm carries an armature, A, which by its motion changes the potential of two electro-magnets. These changes in magnetism cause a bar situated m their magnetic field to reproduce the original vibrations. The ends of the bar are held by magnetic force against two pieces of carbon, c and c These and the bar are included in the primary circuit of an induction-coil. The resistance of the circuit decreases when the bar is drawn up, and increases as the bar descends.
The Microphone.—The device of using several pieces of the semi-conductor instead of one was early tried by Mr. Edison. He found in general that the loudness of the sound was increased by thus multiplying the number of contact-surfaces, but also that the articulation was impaired. Instruments of this nature have since become known as microphones, though it is not probable that faint sounds were ever augmented through their agency. Fig. 9 shows one of the first forms of the microphone, invented by Mr. Edison, April 1, 1877. Four pieces
of charcoal are used, C C, etc., each supported by an upright spring, as at S and S'. The piece of charcoal nearest the diaphragm impinges upon a disk of carbon, which is fastened to the centre of the diaphragm. The primary wires of an induction-coil are attached to the diaphragm and the spring S'. The circuit is then completed through the semi-conductors. Other forms are shown in Figs. 10 and 11. the former has two carbons, separated by a plate of metal. The latter has three contiguous pieces of carbon.
Fig. 12 illustrates a microphone having ten plates of silk, a mixture of dextrine and lampblack having been previously worked into the pores.
In Fig. 13 fifty disks, D, of protoxide of iron, are shown inclosed in a glass tube.
A novel form of transmitter, used by Mr. Edison in his experiments, is shown in Fig. 14. The semi-conductor is a collection of small fragments
of cork, covered with plumbago. It can be used with or without a diaphragm.
The instrument shown in Fig. 15 acts both as a transmitter and receiver. The solid carbon of the transmitter is here replaced by silk fibres coated with graphite. Its action as a receiver is probably due to the attraction of parallel currents; the volume of the whole being contracted during the passage of a current through F.
In the accounts which have been published of experiments with the microphone, the statement has frequently been made. that minute sounds are actually magnified by it, in the same sense that minute objects are magnified by the microscope. A little reflection will show, however, that there is no real analogy in the action of the two instruments. The sound that is heard in the receiving-instrument of the microphone, when a fly is walking across the board on which the transmitter is placed, is not the sound of the fly's footsteps, any more than the stroke of a powerful electric bell, or sounder, is the magnified sound of the operator's fingers tapping lightly, and it may be inaudibly, upon the key. This view of the subject readily explains why the microphone has failed to realize the expectations of many persons, who, upon its first exhibition, enthusiastically announced that by its aid we should be able to hear many sounds in Nature which had hitherto remained wholly inaudible.
Short-circuiting Telephones.—a number of the telephones invented by Mr. Edison may be classed together as short-circuiting or cut-out telephones. The principle upon which they act may be thus briefly stated: In vibrating, the diaphragm cuts from the circuit resistances which are proportional to the amplitude of the vibrations. A transmitter constructed upon this principle is shown in Fig. 16. A lever, L, of metal, vibrating in a vertical plane, rests at one end upon a strip of carbonized silk, C, which is part of the primary circuit of the induction-coil I. In the course of its vibrations the lever cuts from the circuit parts of the silk, the current passing temporarily through the lever.
Another, acting on the same principle, but differing considerably in construction, is shown in Fig. 17. A fine wire, W, of high resistance, is wrapped around a cylinder in a spiral groove.
The wire forms part of the primary circuit of the coil C. A spring, S, of metal, in the form of an ellipse, is fastened at one side to the diaphragm, while the other side presses against the uninsulated wire upon the cylinder. The diaphragm, in moving toward the right, flattens the spring, making it impinge upon a greater number of convolutions than it would if the motion were in the opposite direction. The resistance of the circuit depends, therefore, upon the position of the centre of the diaphragm. The disadvantage of this arrangement is, that either a whole convolution or none at all is suppressed from the circuit, rendering the current rather more intermittent than pulsatory.
In Fig. 18 a similar spring rests upon a narrow strip of metal on the surface of a glass plate. The film is shown in perspective at F, and consists of a fine strip of the silvered surface of a mirror, the rest of the burnished metal having been removed.
The action of this instrument is similar to that of the instrument shown in Fig. 16.
Still another form of short-circuiting telephone is shown in Fig. 19. A spiral spring, W, is wrapped about a cylinder, the diaphragm pressing against the last turn, so that in vibrating the convolutions approach or recede from each other. A very slight motion of the diaphragm is sufficient to cause the first few coils to come together; and in general the number of coils that thus touch each other is dependent upon the amplitude of the diaphragm's motion. The wire is included in the primary circuit of an induction-coil, so that the resistance of the circuit fluctuates as the diaphragm vibrates.
Condenser-Telephones.—Telephones in which static charge, instead of current strength, is made to vary in unison with the vocal utterances, have also been tried with success by Mr. Edison. The forms shown in Figs. 20 and 21 differ only in construction, not in principle.
The former consists of a circular vocalizing chamber, with mouthpiece at V. The chamber is surrounded with plates, which are connected with each other and to the ground. These plates are free to vibrate, and are shown in the figure in section, as at P'. Immediately behind each of these stands a similar plate as at P, held at its centre by an adjusting screw. The outside row of plates is electrically connected with each other and with the battery, which goes to the line.
When the inside row of plates vibrates under the influence of a sound, the distance between the plates varies and changes their static capacity.
In Fig. 21 the plates are arranged as in the ordinary form of a condenser. An initial pressure is put upon them by a screw passing through a portion of the solid frame of the instrument. The diaphragm in vibrating varies the distance between the plates. This alters their static charge, and affects also the electric tension of the line. The resistance of a conductor is dependent upon its shape. If an isometric block of metal be drawn out into a wire, its resistance may be indefinitely increased. This fact lies at the basis of several ingenious telephones invented by Mr. Edison. The one shown in Fig. 22 is of exceedingly simple construction. A globule of mercury, M, rests upon a slightly concave plate of metal. A needle from the diaphragm indents its upper surface, and, as it vibrates, slightly alters the shape of the globule. This alteration, though exceedingly small, is sufficient to vary the resistance of the telephonic current considerably.
It is a peculiar characteristic of a globule of mercury that it changes its original shape during the passage of a current through it. Mr. Edison has made an application of this phenomenon in the telephone-receiver shown in Fig. 23. The globule of mercury, M, is placed, together with a conducting solution, in a V-shaped tube. The currents from a transmitter, passing through the contents of a tube, elongate the mercury. This agitates the liquid and vibrates the float F, which is fastened to the centre of the diaphragm.
The Voltaic Pile Telephone.—We have shown in Fig. 24 an instrument known as the pile-telephone. A piece of cork, K, fastened to the diaphragm, presses upon a strip of platinum which is attached to a plate of copper. The latter is one of the terminal plates of an ordinary voltaic pile. The other terminal plate presses against the metallic frame of the instrument. When the pile is included in a closed telephonic
circuit, it furnishes a continuous current. The strength of this current depends upon the internal resistance of the pile, and the latter is varied by vibrating the diaphragm.
A convenient and peculiar form of receiver used by Mr. Edison is shown in Fig, 25. It is like the ordinary magneto-telephone, except that the circular diaphragm is replaced by a strip of thin iron, the edges having been bent so as to render it stiff. We mention it, simply because it demonstrates the fact that it is not essential that the diaphragm be circular.
A novel and purely mechanical telephone is illustrated by Fig. 26. In place of a line-wire, the illuminating gas, contained in gas-pipes, is used. It is calculated for short distances only, as it is essential that
the gas used in communicating offices should be drawn from the same main pipe. In the figure, P is the main pipe. The telephones are represented at T and T'. The instrument is merely a cone fastened by its apex to the gas-pipe in place of the burner. The larger end is closed by a thin circular diaphragm. The vibrations are conveyed from one diaphragm to another through the medium of the gas.
The phonograph and telephone, when combined, form an instrument known as the telephonograph, of which Fig. 27 is a representation. The drum of the phonograph is shown in section. The diaphragm, instead of being vibrated by the voice, is vibrated by the currents which traverse the helix, H, and which originate at a distant station. The object of the instrument is to obtain a record of what is said at the distant office, which can be converted into sound when desired.
The Motograph.—The motograph-receiver, from which we have been accustomed to hear sounds almost destitute of quality, has, by a little modification, become an articulating telephone. It works quite well in conjunction with the Edison carbon-transmitter. In Fig. 28 the back of the motograph-receiver has been removed, showing its construction. Within the drum D is contained the decomposing solution, and the covering surrounding the drum is kept constantly moist by capillary action. A metallic spring attached to the centre of the diaphragm rests upon the drum; while receiving, the drum is revolved by turning the milled screw at A.
A new transmitter for the motograph is shown m Fig. 29. The point P, projecting from the centre of the diaphragm, impinges upon a wrapping of plumbagoed silk, covering a small drum capable of adjustment by a thumb-screw.
The Carbon-Rheostat.—A very important application of the property possessed by semi-conductors of changing their resistance under varying pressure, is shown in Fig. 30. The cut represents the new Edison carbon-rheostat. The instrument is designed to replace the ordinary adjustable rheostats whenever a resistance is to be inserted in a telegraph-line; as, for example, in balancing quadruplex circuits.
Fig. 31 is a vertical section. It shows a hollow cylinder of vulcanite, containing fifty disks of silk that has been saturated with sizing, and well filled with fine plumbago and dried. These are surmounted by a plate of metal, C, which can be raised or lowered by turning the
The Tasimeter.—Fig. 32 shows in perspective the latest form of the Edison microtasimeter, or measurer of infinitesimal pressure.
The value of the instrument lies in its ability to detect small variations of temperature. This is accomplished indirectly. The change of temperature causes expansion or contraction of a rod of vulcanite, which changes the resistance of an electric circuit by varying the pressure it exerts upon a carbon-button included in the circuit. During the total eclipse of the sun, July 29, 1878, it successfully demonstrated the existence of heat in the corona. It is also of service in ascertaining the relative expansion of substances due to a rise of temperature.
In Fig. 33 the important parts are represented in section, affording an insight into its construction and mode of operation.
The substance whose expansion is to be measured is shown at A. It is firmly clamped at B, its lower end fitting into a slot in the metal
plate, M, which rests upon the carbon-button. The latter is in an electric circuit, which includes also a delicate galvanometer. Any variation in the length of the rod changes the pressure upon the carbon, and alters the resistance of the circuit. This causes a deflection of the galvanometer-needle—a movement in one direction denoting expansion of A, while an opposite motion signifies contraction. To avoid any deflection which might arise from change in strength of battery, the tasimeter is inserted in an arm of the Wheatstone's bridge.
In order to ascertain the exact amount of expansion in decimals of an inch, the screw S, seen in front of the dial, is turned until the deflection previously caused by the change of temperature is reproduced. The screw works a second screw, causing the rod to ascend or descend, and the exact distance through which the rod moves is indicated by the needle, N, on the dial. The instrument can also be advantageously used to measure changes in the humidity of the atmosphere.
In this case the strip of vulcanite is replaced by one of gelatine, which changes its volume by absorbing moisture.
The Arephone.—The arephone, an invention of Mr. Edison's, for amplifying sound, has already attracted considerable attention, though as yet it has not been perfected.
Its object is to increase the loudness of spoken words, without impairing the distinctness of the articulation.
The working of the instrument is as follows: The magnified sound proceeds from a large diaphragm, which is vibrated by steam or condensed air. The source of power is controlled by the motion of a second diaphragm, vibrating under the influence of the sound to be magnified.
There are three distinct parts to the instrument:
A source of power—compressed air.
An instrument to control the power.
A diaphragm vibrating under the influence of the power.
The first of these is usually compressed air, supplied from a tank. It is necessary that it should be of constant pressure.
The second, shown in section at Fig. 34, consists of a diaphragm and mouth-piece, like those used in the telephone. A hollow cylinder is attached by a rod to the centre of the diaphragm. The cylinder, and its chamber, E, will therefore vibrate with the diaphragm.
A downward movement lets the chamber communicate with the outlet H, an upward movement with the outlet G. The compressed air enters at A, and fills the chamber, which, in its normal position, has no outlet. Every downward vibration of the diaphragm will thus condense
the air in the pipe C, at the same time allowing the air in B to escape via F. An upward movement condenses the air in C, but opens I.
The third and last part is shown in section in Fig. 35. It consists of a cylinder and piston, P, like that employed in an ordinary engine.
The piston-rod is attached to the centre of a large diaphragm, D. The pipes C and B are continuations of those designated in Fig. 34 by the same letters. The pipe C communicates with one chamber of the cylinder, and B with the other. The piston, moving under the influence of the compressed air, moves also the diaphragm, its vibrations being, in number and duration, identical with those of the diaphragm in the mouth-piece.
The loudness of the sound emitted through the directing tube, F, is dependent on the size of the diaphragm and the power which moves it. The former of them is made very large, and the latter can be increased to many hundred pounds' pressure.
The Harmonic Engine.—This instrument is shown in Fig. 36. Mr. Edison claims that ninety per cent, of the power derived from the battery
is utilized through its agency. The parts of the machine are: a tuning-fork of large dimensions, vibrating about thirty-five times a second, and carrying on each arm a weight of thirty-five pounds. The amplitude of the vibration is about one-eighth of an inch, and he vibrations are sustained by means of two very small electro-magnets, placed near the end of each arm. These magnets are connected in circuit with each other, and with a commutator worked by one of the arms.
Small branches extend from the fork-arms into a box containing a miniature pump having two pistons, one attached to each arm. Each stroke of the pump raises a very small quantity of water, but this is compensated for by the rapidity of the strokes Mr. Edison proposes to compress air with the harmonic engine, and use it as a motor for propelling sewing-machines and other light machinery. It appears to be considerably m advance of other electric engines, and through its agency electricity may yet become a valuable motive-power.