It is obvious, however, that such methods as these are only capable of transmitting speech over very limited distances, and if greater ranges are to be traversed much more powerful transmitters must be employed for the purpose.
Ernest Ruhmer, after long and laborious researches, finally succeeded in transmitting speech many miles by taking advantage of the speaking arc, discovered by Simon, who observed that an arc lamp gave out a loud rattling noise if its current supply was interfered with. An electric arc consists of two carbon rods, connected to a generator. When the carbons are brought into contact for a moment and then drawn apart to a short distance, a kind of electric flame or arc is produced between the points of carbon, and a brilliant white light is emitted by the white hot points of the carbon electrodes.
Ruhmer immediately made the arc serve as a telephone receiver and speak by utilizing the pulsating current of a telephone transmitter to vary the current supplying the arc.
FIG. 138.—Circuit showing how a singing arc is arranged.
The arc could thus be made to sing, whistle or reproduce music and the human voice perfectly, if the sounds were clearly conveyed into the transmitter. Further investigations showed that every alteration of current caused by the action of the transmitter also caused an alteration in the intensity of the light and radiation of the lamp. The speaking arc could therefore be used as a perfect photophone transmitter by directing the rays toward the receiving station with the aid of a parabolic reflector.
In 1902 Ruhmer performed a series of experiments in Germany on the Wannsee, near Berlin. A large motorboat, the Germania, was fitted with an electric searchlight connected with a microphone, so as to form a speaking arc. The receiving station was located on the shore, so that the distance between the stations could be easily enlarged by moving the boat.
The receptor was an ordinary selenium cell, placed at the focus of a large reflector and connected with a telephone receiver and battery. No difficulty was encountered in reproducing the speech over varying distances up to about three miles.
CHAPTER IX. THE WIRELESS TELEPHONE.
The applications of any of the wireless telephone systems thus far described are very limited, for at the best they only operate under the most favorable conditions, and then over rather limited distances. In the case of any system whereby the speech must be transmitted over a beam of light, the great resulting limitations are that the transmission can only take place in a straight line over water or clear country, and that stormy weather or a fog will interrupt communication.