shaped piece of carbon. A second cup is mounted in a rigid position directly in back of the first. The space between is filled with small polished granules of carbon.

FIG. 131.-Diagram of a telephone transmitter.

When these are in a perfectly free and loose state their resistance to an electric current is very great, and they allow almost none to flow.

When slightly compressed their resistance is greatly lowered, and they permit the current to pass. The vibrations of the diaphragm exert a varying pressure upon the gran

FIG. 132.-Diagram showing the principle and construction of the telephone receiver.

ules, with a corresponding variation in their resistance and the amount of current flowing.

The receiver, as has already been explained, consists of a thin iron disk, placed very near but not quite touching the end of a small bar of steel, permanently magnetized, about which is wound a coil of fine insulated wire.

The

ends of this coil are connected to the wires leading from the transmitter and battery. The varying currents of electricity, produced by the transmitter, generate corresponding changes in the magnetism of the receiving instrument, and thus, by alternately attracting and repelling the diaphragm, cause it to vibrate and emit sounds.

Alexander Graham Bell, the ingenious inventor of the telephone, with the aid of Sumner Tainter was the first who achieved success in the attempts to transmit speech without the aid of connecting wires between the source of intelligence and the receptor.

In 1873 Willoughby Smith announced that the element selenium possesses the abnormal property of changing its electrical resistance under the influence of light. Bell and Tainter took advantage of this discovery, and devised selenium cells, in which selenium is formed into narrow strips between the edges of broad conducting plates of brass. The resistance of the cell in the darkness is approximately twice the resistance when illuminated.

This property of the cell was immediately applied to the

FIG. 135.-Photophone transmitting apparatus using acetylene flame to furnish light.

construction of the photophone, an instrument which transmits sounds to a distance by means of a beam of light reflected to a distant spot from a thin mirror thrown into vibration by the voice. Over fifty different forms were devised but the most successful consisted of a transmitter composed of a glass disk, silvered to reflect a pencil of light focused from the sun, or an arc lamp. This glass disk was used as a diaphragm similar to that of an ordinary telephone transmitter, except that the rear side of it was made free to reflect the beam of light. Bell used for this purpose disks about two inches in diameter and the thickness of ordinary paper. The receiver consisted of a parabolic reflector, with a selenium cell placed at its focus. In series with the cell was placed a battery and telephone receiver.

When the membrane was set into vibration by the sound waves, it became alternately concave and convex, the normally parallel rays of light correspondingly converging and diverging. The receiving station was thus under the influence of light rays of rapidly varying intensity in perfect phase with the vibrations of the voice. The reflector concentrated the rays on the selenium cell, and their varying strength changed its resistance and caused a pulsating current to flow through the receiver and reproduce the speech produced at the transmitter.

In another arrangement employed by Bell and Tainter, they used the rays of a powerful electric arc lamp, and by varying the electric current supplying the arc caused the light to fluctuate and produce the same results at the receiver.

These ingenious inventors also devised a method of transmitting speech called the thermophone. The transmitter remained the same as in the photophone-a thin silvered membrane, or glass diaphragm, stretched across the