The application of selenium cells to the transmission of speech over a beam of light has received mention from time to time in the technical press principally in connection with the experiments of Bell and Taintor or Ruhmer. Ruhmer has succeeded in talking a distance of 4½ miles using a speaking arc at the transmitting station. The arc was mounted in the focus of a parabolic mirror, the microphone being connected inductively to the arc circuit by means of an induction coil. The beam of light was picked up by a second parabolic mirror and focused on a selenium cell connected to a battery and telephone receiver. The voice waves impinging on the microphone will act to vary the intensity of the light beam sent out by the arc and in this manner affect the selenium cell at the receiving station which alters the strength of the current thru the receivers and thus reproduces the words.
For experimental purposes a small incandescent lamp may be used at the transmitting station connected in series with a telephone transmitter and battery. The receiving end comprises a selenium cell with proper battery and sensitive telephone receiver. With a little care in adjustment the simple arrangement will transmit the voice across a darkened room without difficulty.
Another application of selenium made by Ruhmer is the automatic control of light buoys. When such buoys are placed in out of the way places it has been necessary to leave the light burning day and night. To prevent the frequent recharging of the gas reservoir this method required, a selenium cell was arranged to turn the gas on and ignite it at nightfall and to extinguish it again in the morning. The cell is connected to a voltmeter the needle of which moves between two contacts. During the day the needle rests against one contact and the gas is turned off by an electromagnet. At nightfall as the resistance of the cell increases the needle falls back till it touches the other contact and operates an electrical mechanism for turning on the gas and igniting it.
Similar applications only await the perfection of a reliable selenium cell in quantities. They could be used to automatically control street lights and lights on advertising signs located along railroads and similar routes of travel.
On the other hand they could be employed in connection with a suitable recording instrument to register the intensity of the sunlight and thus serve as a valuable adjunct to the weather bureau.
They have been used to a limited extent by Prof. Barnard of Lick Observatory in connection with astronomical work and by Minchin in an automatic detector of comets.
The oft heralded successful transmission of photographs over a wire has yet to become an accomplished fact. Photos have been transmitted by Korn with a fair degree of success using selenium cells but such work awaits the perfect cell. Many other investigators have attempted to solve the problem and have proposed various methods, employing images thrown on ground glass screen or thru a photographic negative, a selenium cell or a bank of cells to be affected by the high and low lights of the picture. At the receiving end the current is used to control a light source by one of the many possible methods to reproduce the picture on sensitized paper. These inventions have been variously termed the Telescope, Telectroscope, Telephot, Telephotograph, etc. However with the exception of a very few they have not progressed beyond descriptive matter, drawings and the most preliminary experiments. One interested in the subject would do well to obtain copies of the various patents issued on the art thru a patent lawyer who will do this for a nominal sum.
A scheme has been proposed to utilize the cells in connection with motion pictures to produce the so called talking pictures. This seems entirely feasible provided some form of telephonic relay is used to amplify the reproduced voice currents. The method suggested is to print the spoken words on the motion picture film alongside the pictures in parallel lines of black and white. To do this a beam of light is focused on the film while the picture is being taken, the light being controlled electrically by microphones concealed near the actors. The words spoken acting to vary the intensity of the light, which variations will be printed on the film.
In reproducing the picture a light is arranged to pass thru the voice strip and fall on a selenium cell. In this manner the various shades of black and white imprinted on the film will cause variations in the resistance of the cell and loud talking telephones connected to the cell thru a telephonic relay will reproduce the words. This method has the decided advantage of absolute synchronism lacking in all mechanical devices ever developed for the purpose.
In an attempt to enable the blind to read printed matter by sound Prof. F. C. Brown has devised what is termed a Phonoptican. In his device three or four tiny selenium cells are mounted in a row. The length of the row is equal to the height of the printed letters. Each cell is connected to a telephone receiver thru a separate interrupter. For convenience the telephone receiver used contains as many separate coils and diaphragms as there are cells used. It will be clear that when light falls on one of the cells a certain note will be heard in the receiver and after a little practice the cell affected can be instantly determined.