Wireless Telegraphy and Telephony

 122 WIRELESS TELEGRAPHY 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 investiga- tions 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 sta- tion with the aid of a parabolic reflector. In 1902 Ruhmer performed a series of experiments in Germany on the Wannsee, near Berlin. A large motor- boat, 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.