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 Ridger have used a lamp like this
as a receiver," he continued. "They found that words spoken were
reproduced in the lamp. The telephonic current variations
superposed on the current passing through the lamp produce
corresponding variations of heat in the filament, which are
radiated to the glass of the bulb, causing it to expand and
contract proportionately, and thus transmitting vibrations to the
exterior air. Of course, in sixteen-and thirty-two-candle-power
lamps the glass is too thick, and the heat variations are too
feeble."
Who was it whose voice Brixton had recognised as familiar over
Kennedy's hastily installed detectaphone? Certainly he must have
been a scientist of no mean attainment. That did not surprise me,
for I realised that from that part of Europe where this mystical
Red Brotherhood operated some of the most famous scientists of the
world had sprung.
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