Kennedy shook his head. “No,” he replied, calmly, “no outsider knows a thing about this. You see, I’m not using any ordinary means to prepare against the expert who has brought this situation about. The messages that I am receiving are coming over what we call the ‘phantom circuit.’”
“‘The phantom circuit?’” I repeated, mystified.
“Yes. It seems fantastic at first, I suppose,” he pursued, “but, after all, it is in accordance with the laws of electricity. They know nothing and they cannot cut us off or interfere. You see, I am taking advantage of the fact that additional telephones or so-called phantom lines can be superposed on existing physical lines. It is possible to obtain a third circuit from two similar metallic circuits by using for each side of this third circuit the two wires of each of the other circuits in multiple. All three circuits are independent, too.”
He was growing more and more impatient. Apparently there was some delay at the other end.
“The third telephone current,” he went on, hurriedly, covering up his nervousness by talking about his machine, “enters the wires of the first circuit, as it were, and returns along the wires of the second circuit. There are several ways of doing it. One is to use retardation or choke-coils, bridged across the two metallic circuits at both ends, with taps taken from the middle points of each. But the better method, I think, is the one you have seen me install. I have introduced repeating coils into the circuits at both ends. Technically, the third circuit is then taken off from the mid-points of the secondaries or line windings of these repeating coils. I don’t know what’s the matter,” he added, calling vainly for Shelby Maddox. “Oh, all right. Yes, I’ll wait. But hurry, please.”
I could appreciate Kennedy’s eagerness, for below on the street the tumult was rising.
“It’s working all right,” he reassured. “I suppose you know that the current on a long-distance line is alternating in character, and it passes readily through a repeating coil. The only effect it has on the transmission is slightly reducing the volume. The current passes into the repeating coil, then divides and passes through the two line wires. At the other end, the halves balance, so to speak. Thus currents passing over a phantom circuit don’t set up currents in the terminal apparatus of the side circuits. Consequently, a conversation carried on over the phantom circuit will not be heard on either side circuit, nor does a conversation on one side circuit affect the phantom. You get three messages at once on two sets of wires. We can all talk at once without interfering with one another.”
At any other time I should have been more than interested, but just now the delay was galling. “What’s the trouble?” I inquired.
Kennedy shook his head. “Shelby is talking to Winifred about something. I can hear only a word now and then. But he said it was important and asked me to hold the wire. Evidently she wants to do something he doesn’t want her to do. Yes—hello—yes, this is Kennedy. Say, you’ll have to— Oh, good morning, Miss Walcott. Yes, fine. What? Why—certainly—if he says so, you may. That’s right. Go right ahead. I am attending to everything at this end now.”
A moment later when Craig restored the telephone to its normal condition he looked at me with a smile.