the Osram lamp in the ceiling. Was he, too, crazy?

"Here, Mr. Brixton, take these two receivers of the detectaphone,"
said Kennedy. "Tell me whether you can recognise the voice."

"Why, it's familiar," he remarked slowly. "I can't place it, but
I've heard it before. Where is it? What is this thing, anyhow?"

"It is someone hidden in the storeroom in the basement," answered
Craig. "He is talking into a very sensitive telephone transmitter
and--"

"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,