n the extra special
apparatus.

I implied a moment ago why this system is called a "carrier-current"
system; it is because "the high-frequency currents carry in their
modulations the speech significance." Sometimes it is called a system of
"multiplex" telephony because it permits more than one message at a
time.

This same general principle is also applied to the making of a multiplex
system of telegraphy. In the multiplex telephone system we pictured
transmitting and receiving sets very much like radio-telephone sets. If
instead of transmitting speech each transmitter was operated as a C-W
transmitter then it would transmit telegraph messages. In the same
frequency range there can be more telegraph systems operated
simultaneously without interfering with each other, for you remember how
many cycles each radio-telephone message requires. For that reason the
multiplex telegraph system which operates by carrier-currents permits as
many as ten different telegraph messages simultaneously.

You remember that I told you how capacity effects rob the distant end of
a pair of wires of the alternating current which is being sent to them.
That is always true but the effect is not very great unless the
frequency of the alternating current is high. It's enough, however, so
that every few hundred miles it is necessary to connect into the circuit
an audion amplifier. This is true of carrier currents especially, but
also true of the voice-frequency currents of ordinary telephony. The
latter, however, are not weakened, that is, "attenuated," as much and
consequently do not need to be amplified as much to give good
intelligibility at the distant receiver.

[Illustration: Fig 135]

In a telephone circuit over such a long distance as from New York City
to San Francisco it is usual to insert amplifiers at about a dozen
points along the route. Of course, these amplifiers must work for
transmission in either direction, amplifying speech on its way to San
Francisco or in the opposite direction. At each of the amplifying
stations, or "repeater stations," as they are usually called, two vacuum
tube amplifiers are used, one for each direction. To connect these with
the line so that each may work in the right direction there are used two
of the bridges or resistance squares. You can see from the sketch of
Fig. 135 how an alternating current from the east will be amplified and
sent on to the west, or vice versa.

[Illustration: Fig 136]