"The phantom circuit?" repeated Brixton, chafing at the delay.

"Yes, it seems fantastic at first, I suppose," pursued Kennedy
calmly; "but, after all, it is in accordance with the laws of
electricity. It's no use fretting and fuming, Mr. Brixton. If
Janeff can wait, we'll have to do so, too. Suppose we should start
and this Kronski should change his plans at the last minute? How
would we find it out? By telepathy? Believe me, sir, it is better
to wait here a minute and trust to the phantom circuit than to
mere chance."

"But suppose he should cut the line," I put in.

Kennedy smiled. "I have provided for that, Walter, in the way I
installed the thing. I took good care that we could not be cut off
that way. We can hear everything ourselves, but we cannot be
overheard. He knows nothing. You see, I took 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.

"The third telephone current 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 more desirable method is the one you saw me install