It seemed to consist of two rods much like electric light carbons, fixed horizontally in a wooden support with a spindle-shaped bit of carbon between the two ends of the rods. Wires were connected with binding screws at the free ends of the carbon rods.
First Craig made a connection with an electric light socket from which he removed the bulb, cutting in a rheostat. Then he attached the free wires from the carbons to a sort of telephone headgear and switched on the current.
"What is it?" I asked curiously.
"A geophone," he replied simply.
"And what is a geophone?" I inquired.
"Literally an earth-phone," he explained. "It is really the simplest form of telephone, applied to the earth. You saw what it was. Any high school student of physics can make one, even with two or three dry batteries in circuit."
"But what does it do?" I asked.
"It is really designed to detect earth vibrations. All that is necessary is to set the carbon stick arrangement, which is the transmitter of this telephone, on the floor, place myself at the other end and listen. A trained ear can readily detect rumblings. Really it is doing in a different and often better way what the seismograph does. This instrument is so sensitive that it will record the slamming of a cellar door across the street. No one can go up those stairs next door without letting me know it, no matter how cautious he is about it."
Craig stood there some minutes holding the thing over his ears and listening intently.
"The vibrodyne machine isn't running," he remarked finally after repeated adjustments of the geophone. "But someone is in that little room under Creighton's workshop. I suspected that something was down there after that watch crystal test of mine. Now I know it. I wonder what the man is doing?"