“There’s no danger,” said Drew. “Come here, Miss Stockbridge,” he added. “I want to show you what was all ready for you.”
The detective raised the hard-rubber receiver. “Here we have the diaphragm,” he said, pointing. “It’s a round plate of soft iron. It’s secured to the rubber by an insulated ring. It is the part you press up to your ear when you listen at a telephone. There’s a small hole punched in this one. The same sized hole extends down through the center core, or magnet. This hole isn’t rifled. It couldn’t well be rifled save with special machinery. That’s why the bullet found in Mr. Stockbridge’s brain was without longitudinal scorings. It was fired from a smooth-bored pistol.”
“That’s what you thought!” blurted Delaney with loyalty.
“I was at sea,” said Drew. “Now,” he continued, “we have a live cartridge at the opposite end of this core from the diaphragm. See it?” Loris leaned over the little table.
“Right here!” The detective pointed. “That is a twenty-two cartridge with a cupronickle bullet. See the cap? See how it is held from coming back by those tiny screws about the rim?”
Loris nodded and gathered up her straying hair.
“Now,” continued Drew. “Now, this cartridge was exploded by the action of the human voice. Here’s a tiny spiral of very slender platinum wire. It must be number forty, at least. That’s very fine! This spiral is in series with the winding about the magnets. The same current pulsated by the human voice which actuates the receiver diaphragm, also passed through this spiral. Now,” Drew paused. “Now,” he added with rising voice, “here is a tiny charred piece of match-head, I guess. It was set in the coil. It flared when the wire became hot. The heat was sufficient to ignite the cap. See it!”
“I see it!” exclaimed Nichols.
“The action is simple,” continued Drew. “A pulsation of the current which was formed by the action of the vibrating, transmitter diaphragm, also pulsated the fine wire before it went to the receiver magnets. The louder the voice into the transmitter the more current—measured in fractions of amperes—passed through the spiral. It became sufficiently hot to flare the piece of match-head or whatever Cuthbert placed there. This flare was communicated to the percussion cap, or fulminate of mercury, at the base of the cartridge. This exploded the powder charge, which in turn projected the cupronickle bullet forward through the tube or the bore of the receiver and out through the thin, metal diaphragm, and––”
“What’s that?” asked the operative.