This property of the cell was immediately applied to the 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 back of a mouthpiece, and arranged to reflect the rays of the sun, or the light of an arc lamp.
The receiver was a small glass bulb containing a plate of mica covered with lampblack, or little charred pieces of cork. The glass bulb was placed in the focus of a reflector, which collected the rays and concentrated them. The variations in the intensity of the heat radiations caused the air in the bulb to expand or contract with each vibration. Rubber tubes extended from the bulb to the ears of the observer, and the pulsations of air, traveling through the tube as sound waves, would strike the ear-drum and reproduce the speech.
FIG. 136.—Powerful searchlight arranged to transmit speech over a beam of light.
Both of these methods were later very much improved by the employment of Koenig's manometric flame in place of the silvered mirror as a transmitter. As explained in the last chapter, speech delivered into the mouthpiece causes the gas to become compressed or rarefied in direct accordance with the sound waves, and the flame rises and falls with a rapidity too great to be detected by the naked eye. These rapid alterations in the intensity of the illumination of the flame act on the selenium cell, and reproduce the original voice perfectly in the telephone receiver.
FIG. 137.—The Electric arc.