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electric resistance under pressure. The greater the pressure on it the lower becomes its resistance, and Edison's idea was to cause the pressure of the sound waves of the voice to effect this change of resistance,
knowing as he did that the diminished resistance due to a stronger pressure would allow more current to pass through the carbon, and thus the undulations of the current caused by the vibrations of the voice would be in keeping with the sound. He therefore contrived a mouth-piece to speak into, and having across it a drum, or tympan, of metal or mica, D, to vibrate to and
fro under the voice like the Fig. 31.
disc of Bell. This tympan
pressed at its middle against an india-rubber buffer A, which conveyed the pressure to a wafer of very fine lamp-black c made of compressed kerosene smoke. An adjusting screw was brought to: bear on this wafer behind in order to regulate the presented sure on it to begin with; and wires connected the wafer between the battery and line. On speaking into the mouthpiece, the sound waves impinged on the tympan, which communicated its fluctuating pressure to the lamp-black behind, and thereby varied the strength of current flowing through it to the line in accordance with that pressure. The undulatory current thus produced traversed the line, and passing
through the receiving instrument reproduced the original sounds.
Edison's receiving instrument was also different from Bell's. In fact, this bold inventor had determined to invent a telephone which would not only be entirely different from Bell's, but would do what Bell's could not do, and talk out loudly. Bell's telephone could only be heard at all with the ear held close to the earpiece; Edison wished to make a room full of people hear if necessary. That this property was not in itself a very desirable one in a telephone for ordinary use will be admitted by most people who have to carry on a private and confidential parley about business matters. However, Edison set himself the task, and we cannot but admire the hardihood of genius and the mechanical skill which enabled him to accomplish his purpose with materials so unpromising.
The action of the Edison receiver will be understood from Fig. 32, which is a diagram of the telephone circuit.
Here A is the carbon transmitter which we have just described, connected up to the line indirectly through the primary circuit of an induction coil; the secondary circuit of which is connected to the line on the one side and the earth” on the other. The distant end of the line is of course connected to “earth” through the receiver. This consists of a mica plate D D, fixed round its edge and free to vibrate at its centre, from which projects a metal arm b to which the end of the line is joined. To the outer end of this arm is attached a small pin or style of platinum P, which presses on the surface of a chalk cylinder or barrel cc, mounted on an axle and capable of being revolved in the direction of the arrows by turning a handle. The axle of the cylinder is connected to “earth” so that the current arriving from the line passes down the style through the cylinder to“ earth.” The cylinder is made by compressing precipitated chalk in a hydraulic press, and formerly it was moistened with a solution of potassium hydrate and mercuric acetate, but now we believe it is simply wetted with distilled water. The surface is turned in a lathe so as to be very smooth and exert a delicate friction on the platinum style as the cylinder is revolved.
This friction tends to drag the style forward after the cylinder, and thus bends the mica plate inwards following the barrel. But when the electric current traverses the contact between the style and chalk, the friction is reduced and the style slips back, allowing the mica plate to partially recover its old posture. The fall of friction being proportional to the strength of current, it follows that the style will slip to and fro on the surface of the chalk in accordance with the modulations of the vocal currents in the line, and the mica plate, thus kept in vibration, will reproduce the original sounds addressed to the transmitter.
Edison's receiver as actually made is shown in Fig. 33, where A is the chalk cylinder mounted on an axle B, supported by the frame , and rotated by the handle w through a toothed gearing; while g g is a shaft carrying a small pad which dips into a water vessel T. On turning this shaft the pad is pressed against the chalk to moisten it. The style is shown touching the
surface of the chalk and carried by an arm c which is seen endwise.
The lower end of the arm is attached to the centre of the mica plate D. A spring s adjusted by the screw E, serves to press the style against the chalk. One drawback of this instrument is the necessity of turning the handle in order to hear. Another is the tendency
of the chalk to dry and the trouble of keeping it moist. The instrument speaks loud enough for a roomful of people to hear and with a punchinello twang, but quite distinct enough to understand, and all kinds of music, especially the notes of a bell, are rendered by it with admirable fidelity.
The invention of the microphone, and its use as a transmitter to the Bell receiver, has, however, put the Edison telephone out of competition. In the winter of 1877--8, Professor D. E. Hughes, the inventor of the Hughes type-printing telegraph, discovered that a delicate contact between two conductors or semiconductors of electricity could be made to transmit sound and speech, by sending an electric current through it in circuit with a receiving telephone.
Thus, if three French nails A are arranged as in Fig. 34, so that the current from a battery B traverses the