am which
flows through the Z-side of the square goes out to station 2. At this
station the electrons have three paths between _d_ and _b_. I have
marked these by arrows and you see that one of them is through the
receiver. The current which is started by the transmitter at station 1
will therefore operate the receiver at station 2 but not at its own
station. Of course station 2 can talk to 1 in the same way.

The actual set-up used by the telephone company is a little different
from that which I have shown because it uses a single common battery at
a central office between two subscribers. The general principle,
however, is the same.

[Illustration: Fig 132]

It won't make any difference if we use equal inductance coils, instead
of the R-resistances, and connect the transmitter to them inductively as
shown in Fig. 132. So far as that is concerned we can also use a
transformer between the receiver and the points _c_ and _d_,
as shown in the same figure.

[Illustration: Fig 133]

We are now ready to put in radio equipment at station 2. In place of the
telephone receiver at station 2 we connect a radio transmitter. Then
whatever a person at station 1 says goes by wire to 2 and on out by
radio. In place of the telephone transmitter at station 2 we connect a
radio receiver. Whatever that receives by radio is detected and goes by
wire to the listener at station 1. In Fig. 133 I have shown the
equipment of station 2. There you have the connections for wire to radio
and vice versa.

One of the most interesting developments of recent years is that of
"wired wireless" or "carrier-current telephony" over wires. Suppose that
instead of broadcasting from the antenna at station 2 we arrange to have
its radio transmitter supply current to a wire circuit. We use this same
pair of wires for receiving from the distant station. We can do this if
we treat the radio transmitter and receiver exactly like the telephone
instruments of Fig. 132 and connect them to a square of resistances. One
of these resistances is, of course, the line between the stations. I
have shown the general arrangement in Fig. 134.

You see what the square of resistances, or "bridge" really does for us.
It lets us use a single pair of wires for messages whether they are
coming or going. It does that because it lets us connect a transmitter
and also a receiver to a single pair of wires in such a way that the
transmitter can't affect the receiver. Whatever the t