ransmitter sends
out goes along the wires to the distant receiver but doesn't affect the
receiver at the sending station. This bridge permits this whether the
transmitter and receiver are radio instruments or are the ordinary
telephone instruments.

[Illustration: Fig 134]

By its aid we may send a modulated high-frequency current over a pair of
wires and receive from the same pair of wires the high-frequency current
which is generated and modulated at the distant end of the line. It lets
us send and receive over the same pair of wires the same sort of a
modulated current as we would supply to an antenna in radio-telephone
transmitting. It is the same sort of a current but it need not be
anywhere near as large because we aren't broadcasting; we are sending
directly to the station of the other party to our conversation.

If we duplicate the apparatus we can use the same pair of wires for
another telephone conversation without interfering with the first. Of
course, we have to use a different frequency of alternating current for
each of the two conversations. We can send these two different modulated
high-frequency currents over the same pair of wires and separate them by
tuning at the distant end just as well as we do in radio. I won't sketch
out for you the tuned circuits by which this separation is made. It's
enough to give you the idea.

In that way, a single pair of wires can be used for transmitting,
simultaneously and without any interference, several different telephone
conversations. It takes very much less power than would radio
transmission and the conversations are secret. The ordinary telephone
conversation can go on at the same time without any interference with
those which are being carried by the modulations in high-frequency
currents. A total of five conversations over the same pair of wires is
the present practice.

This method is used between many of the large cities of the U. S.
because it lets one pair of wires do the work of five. That means a
saving, for copper wire costs money. Of course, all the special
apparatus also costs money. You can see, therefore, that this method
wouldn't be economical between cities very close together because all
that is saved by not having to buy so much wire is spent in building
special apparatus and in taking care of it afterwards. For long lines,
however, by not having to buy five times as much wire, the Bell Company
saves more than it costs to build and maintai