The Yehudi Principle

I am going crazy.
Charlie Swann is going crazy, too. Maybe more than I am, because it was his dingbat. I mean, he made it and he thought he knew what it was and how it worked.
You see, Charlie was just kidding me when he told me it worked on the Yehudi principle. Or he thought he was. “The Yehudi principle?” I said.
“The Yehudi principle,” he repeated. “The principle of the little man who wasn’t there. He does it.”
“Does what?” I wanted to know.
The dingbat, I might interrupt myself to explain, was a head-band. It fitted neatly around Charlie’s noggin and there was a round black box not much bigger than a pillbox over his forehead. Also there was a round flat copper disk on each side of the band that fitted over each of Charlie’s temples, and a strand of wire that ran down behind his ear into the breast pocket of his coat, where there was a little dry cell battery.
It didn’t look as if it would do anything, except maybe either cure a headache or make it worse. But from the excited look on Charlie’s face, I didn’t think it was anything as commonplace as that.
“Does what?” I wanted to know.
“Whatever you want,” said Charlie. “Within reason, of course. Not like moving a building or bringing you a locomotive. But any little thing you want done, he does it.”
“Who does?”
“Yehudi.”
I closed my eyes and counted to five, by ones. I wasn’t going to ask, “ Who ’ s Yehudi? ”