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CHAPTER V.

THE TELEPHONE AND MICROPHONE.

THE speaking telephone of Professor Graham Bell has to a large extent superseded the telegraph instruments formerly employed on private lines for special correspondence. This electrical marvel was invented

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Directly over this pole and

in 1876, and first exhibited at the Centennial Exhibition in Philadelphia that year. It was originally feeble in its effects, owing to the transmitter and the receiver being identical. This apparatus is illustrated in Fig. 30, where the internal construction and external appearance are both shown. It consists of a bar magnet м, contained within the stem and having a small bobbin c of silkcovered copper wire surrounding its upper pole. bobbin is placed a circular

disc D of thin sheet iron, fixed round its edges by the

wooden mouth-piece o, but free to vibrate to and fro at its middle. The ends of the coil c are connected by two wires running outside the stem to terminals T T at its lower end, and connecting wires w w run from these outside the instrument and connect it in circuit with the line wire.

Formerly two such instruments were simply joined up to the line and the earth, and no battery was employed, for on speaking into the mouth-piece of one the vibrations of the voice bent the iron disc to and fro in front of the magnetic pole, and thus, by magnetoelectric induction, set up undulatory currents of electricity in the little coil around the pole. These currents were of a strength and form dependent on the vibrations of the voice, and after travelling along the line they passed through the coil of the receiving instrument, where by a reverse action they set the iron disc into a vibration sympathetic with the vibration of the disc of the transmitting instrument. The result was that an ear placed at the mouth-piece of that instrument could hear a feeble imitation of the distant speaker's voice.

It was, however, very feeble, owing to the fact that the voice itself generated the electric current; and louder effects were obtained by Edison, who adopted the principle of causing the voice only to vary the strength of a battery current flowing in the line, just as a miller regulates the stream of water flowing in the race by working the sluice valve up and down.

He did this by the "carbon transmitter " shown in section in Fig. 31. There there is no magnetism employed, and the strength of current is regulated by passing it through a wafer of carbon, which alters its

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,

FIG. 31.

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 disc of Bell. This tympan

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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 pres 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.

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Here A is the carbon transmitter which we have just described, connected up to the line indirectly through

F

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 c c, 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.

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