Scarcely a word was spoken as hour after hour Craig sat with the
receiver to his ear, connected with the coils down in the
storeroom. "You might call this an electric detective," he had
explained to Spencer. "For example, if you suspected that anything
out of the way was going on in a room anywhere this would report
much to you even if you were miles away. It is the discovery of a
student of Thorne Baker, the English electrical expert. He was
experimenting with high-frequency electric currents, investigating
the nature of the discharges used for electrifying certain things.
Quite by accident he found that when the room on which he was
experimenting was occupied by some person his measuring-
instruments indicated that fact. He tested the degree of variation
by passing the current first through the room and then through a
sensitive crystal to a delicate telephone receiver. There was a
distinct change in the buzzing sound heard through the telephone
when the room was occupied or unoccupied. What I have done is to
wind single loops of plain wire on each side of that room down
there, as well as to wind around the room a few turns of concealed
copper wire.
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