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 maintain 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: Fig 136]

There are a large number of such repeater stations in the United States along the important telephone routes. In Fig. 136 I am showing you the location of those along the route of the famous "transcontinental telephone-circuit." This shows also a radio-telephone connection between the coast of California and Catalina Island. Conversations have been held between this island and a ship in the Atlantic Ocean, as shown in the sketch. The conversation was made possible by the use of the vacuum tube and the bridge circuit. Part of the way it was by wire and part by radio. Wire and radio tie nicely together because both operate on the same general principles and use much of the same apparatus.

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