“Also, you may have heard that the FBI has an electronic gadget so sensitive that it can eavesdrop on the conversations of crooks, even though they may be sitting in a boat half a mile from shore.”
“I’ll bet the Shoshonean water spirits take a dim view of that,” said Ralph, grinning.
Donovan waved him to silence with his pipe and continued.
“Now my guess is that Cavanaugh is using a lot of power from a portable generator to produce a beam bright enough to be seen a hundred or so miles away. And it’s a lot easier for him to modulate that current so it will modulate the beam than to use revolving mirrors or some other mechanical means to do the job. There is bound to be considerable leakage in a circuit of that kind. I think I can go to one of the radio supply stores in Farmington tomorrow and pick up enough parts to make an electronic ‘ear’ that can tune in on that leakage if we get it within a hundred feet of Cavanaugh’s transmitter.”
“Sherlock Donovan,” said Ralph, “I take off my hat to you.”
The haywire “ear” that Donovan built during the next several days with what little assistance Sandy was able to supply didn’t look like much. It was just a collection of transistors, fixed and variable condensers, coils and verniers mounted on an old breadboard. But it had the advantage of being light and portable. And, when they tried it out with the help of their radio receiving set, it worked!
They found that, with the set’s loudspeaker disconnected, they could place their gadget several hundred feet away and hear the programs perfectly, either on the short-wave or regular broadcasting channels.
“That does it,” Donovan finally said after a careful series of night tests. “We don’t know the frequency that Cavanaugh is using as a modulator, but this thing is flexible enough to tune in on practically any wave band. Now the question becomes, when do we try it out?”
“Why not right now?” Ralph asked.
“Boyd has gone in to town, so I’m in charge of keeping an eye on the well,” said the geologist. “I can’t go with you tonight.”