Wireless Telegraphy and Telephony

WIRELESS TELEGRAPHY 119 construction of the photophone, an instrument which trans- mits 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 ordi- nary telephone transmitter, except that the rear side of it was macle 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 nor- mally parallel rays of light correspondingly converging and diverging'. The receiving' station was thus under the in- fluence of light rays of rapidly varying intensity in perfect phase .with the vibrations of the voice. The reflector con- centrated the rays on the selenium cell, and their varying strength changed its resistance and caused a pulsating cur- rent 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 transmit- ter remained the same as in the photophone a thin sil- vered membrane, or glass diaphragm, stretched across the