What he was looking for neither of us could guess, but his caution had been emphatic enough to halt any question we might have.
Over and over he passed the free end of the apparatus along wall and floor. At each stop he seemed to be considering something carefully, then with a negative nod to himself went on.
This had been going on for some fifteen minutes, when he stopped in the corner back of a coat-tree. He looked about as he pulled the thing from his ear, saw a heavy pair of shears on Hastings’s desk, and seized them.
Deliberately he dug into the plaster of the wall, while Hastings and I bent over anxiously.
He had not gone half an inch before he began to scrape very carefully, as though he were afraid of hurting something alive.
I looked. There, in the wall, back of the plaster, hung a little telltale black disk. I recognized it the instant I saw it and turned quickly, just in time to prevent a question from Hastings.
Some one had been using the detectaphone against us!
Though not a word was said, I realized vaguely what Kennedy later explained. He had suspected it and had made use of a method of finding pipes and metals electrically, when concealed in walls under plaster and paper.
It was a special application of the well-known induction balance principle. One set of coils on the magnet bar received an alternating or vibrating current. The other was connected with a little sensitive telephone. Craig had first established a balance so that there was no sound in the telephone. When the device came near metal piping the balance was destroyed and a sound was heard in the telephone. He had located all the water, steam, and other pipes, the wires from the telephone, the messenger-call box, and other things. There still remained one other pair of wires unaccounted for. The balance had located their existence and exact position. Clever though the installation of the little mechanical eavesdropper had been as an aid to crime, Craig’s detectaphone detector had uncovered it!
Impulsively I seized at the devilish little black disk that had forewarned some one and had nearly cost me my life. I started to yank at it. The wires yielded their slack, but before I could give them a final pull Kennedy grasped my hand.