Ed inserted the lump of phosphor back in the tube, pumped it and restarted the whole gear.
It vibrated again, ever so faintly, against the bottom of the glass. Bronson listened carefully, his engineer's mind trying to identify the sound. It was not the sixteen kilocycle sweep circuit—not the one that scanned the face of the television tube, because this was not a complete set-up and there was no scanning energy necessary. It was vaguely familiar.
It came and it went, that faint vibration. Sometimes it rattled violently, other times it purred gently. Always very faintly of course—for the term 'violently' means only by comparison.
Ed adjusted the field strength of the focusing magnet about the neck of the tube and the vibration strengthened to a noticeable degree. He juggled the controls but found he had hit the maximum or optimum response.
There was something about it.... It was like human whisperings too faint to be understood but not too faint to be unheard. Like the bloop-bleep of a leaky faucet that seems to be saying things about you just too quietly to be really understood. Like the imagined whisperings heard by the paranoiac....
Ed laughed. Hearing things!
Like hades he was hearing things. It was really there. The lump of phosphor moved a perceptible amount as a peak of rattle passed. And yet....
Ed Bronson uncoiled his wiry six feet from the chair and cracked the seal on the tube again. He lifted the top and squinted at the crystalline whiteness that had been rattling so maddeningly.
He went to a cupboard at the end of his laboratory and rummaged among small boxes that stood on one shelf—no two boxes seeming to be of the same size. The upshot of this rummaging was that Bronson had to spend some time repiling the boxes after he had found the contact microphone he was seeking. Eventually, however, Ed Bronson was repumping the tube.
Inside was the crystal phosphor and fastened to it was a sensitive contact microphone.