He left, and returned in a few minutes with the tube. They busied themselves with the connections, and then Don applied the power. Nothing happened. "Run a line from the output back through a voltage-dividing circuit to the in-phase anode," suggested Walt. "How much?" "Put a potentiometer in it so we can vary the amount of voltage. After all, Barney Carroll said that the application of voltage in phase with the transmitted power is necessary to the operation of the relay tube. In transmission of D. C., it is necessary to jack up the in-phase anode with a bit of D. C. That's in-phase with a vengeance!" "What you're thinking is that whatever this sub-level energy is, some of it should be applied to the in-phase anode?" "Nothing but." The cabinet provided a standard potentiometer, and as Don advanced the amount of fed-back voltage, Freddy's voice came booming in louder and louder. It overloaded the audio amplifier, and they turned the gain down as Channing increased the in-phase voltage more and more. It passed through a peak, and then Don left the potentiometer for maximum. "Wes," he said, "call Freddy and tell him to take off for Terra, at about 4-G. Have the gang upstairs hang a ship beam on him so we can follow him with suggestions. Too bad we can't get there immediately." "What I'm worrying about is the available gain," said Wes. "That thing may have given us a gain of a couple of thousand, but that isn't going to be enough. Not for planet-to-planet service." "Later on we may be able to hang a couple of those things in cascade," suggested Walt. "Or if not, I know a trick that will work--one that will enable us to get a gain of several million." "Yeah? Mirrors, or adding machines? You can't make an audio amplifier of a three million gain." "I know it--at least not a practical one. But, we can probably use our audio modulator to modulate a radio frequency, and then modulate the driver with the RF. Then we hang a receiver on to the detector gadget here, and collect RF, modulated, just like a standard radio transmission, and amplify it at RF, convert it to IF, and detect it to AF. Catch?" "Sure. And that gives me another thought. It might just be possible, if your idea is possible, that we can insert several frequencies of RF into the tube and hang a number of receivers on the detector, here." Arden laughed. "From crystal detection to multiplex transmission in ten easy lessons." * * * * * "Call Charley and have him begin to concoct an RF stage for tube-modulation," said Don. "It'll have to be fairly low--not higher than a couple of megacycles so that he can handle it with the stuff he has available, but as long as we can hear his dulcet voice chirping that 'one, two, three, four, test,' of his, we can also have ship-to-Station two-way. We squirt out on the ship beam, and he talks back on the driver transmitter."