Selenium in the dark is a poor conductor of electricity; in
the light it, strange to say, becomes a good conductor. Variations
of light can thus be transmuted into variations of sound. That
pushed-in end of the box which we hid over in Warrington's had, as
you might have noticed, a selenium plate on the inside partition,
facing the open end of the box."
"I understand," I agreed, vaguely.
"Now," he went on, "this property of selenium is used for
producing or rather allowing to be transmitted an electric current
which is interrupted by a special clockwork interrupter, and so is
made audible in this wireless telephone receiver which I have here
connected with this second box. The eye is replaced by the ear as
the detector of light--that is all."
It might have been all, but it was quite wonderful to me, even if
he spoke of it so simply. He continued to adjust the thing as he
talked.
"The clockwork has been wound up by means of a small handle, and I
have moved that rod along a slit until I heard a purring sound.
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