"But the voice—here?" interrupted Brixton impatiently.
Kennedy pointed to the incandescent lamp in the ceiling. "The incandescent lamp," he said, "is not always the mute electrical apparatus it is supposed to be. Under the right conditions it can be made to speak exactly as the famous 'speaking-arc,' as it was called by Professor Duddell, who investigated it. Both the arc-light and the metal-filament lamp can be made to act as telephone receivers."
It seemed unbelievable, but Kennedy was positive. "In the case of the speaking-arc or 'arcophone,' as it might be called," he continued, "the fact that the electric arc is sensitive to such small variations in the current over a wide range of frequency has suggested that a direct-current arc might be used as a telephone receiver. All that is necessary is to superimpose a microphone current on the main arc current, and the arc reproduces sounds and speech distinctly, loud enough to be heard several feet. Indeed, the arc could be used as a transmitter, too, if a sensitive receiver replaced the transmitter at the other end. The things needed are an arc-lamp, an impedance coil, or small transformer-coil, a rheostat, and a source of energy. The alternating current is not adapted to reproduce speech, but the ordinary direct current is. Of course, the theory isn't half as simple as the apparatus I have described."
He had unscrewed the Osram lamp. The talking ceased immediately.
"Two investigators named Ort and Bidger have used a lamp like this as a receiver," he continued.
