"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.
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