ies of the side-bands. At 100,000
cycles we need only the range of wave-lengths between 2940 m. and 3060
m. If the radio-frequency is 500,000 cycles we need a still smaller
range of wave-lengths to transmit the necessary side-bands. Then the
range is from 598 m. to 603 m.

In the case of the transmission of speech by radio we are interested in
having no interference from other signals which are within 2000 cycles
of the frequency of our radio-current no matter what their wave-lengths
may be. The part of the wave-length range which must be kept clear from
interfering signals becomes smaller the higher the frequency which is
being modulated.

You can see that very few telephone messages can be sent in the
long-wave-length part of the radio range and many more, although not
very many after all, in the short wave-length part of the radio range.
You can also see why it is desirable to keep amateurs in the short
wave-length part of the range where more of them can transmit
simultaneously without interfering with each other or with commercial
radio stations.

There is another reason, too, for keeping amateurs to the shortest
wave-lengths. Transmission of radio signals over short distances is best
accomplished by short wave-lengths but over long distances by the longer
wave-lengths. For trans-oceanic work the very longest wave-lengths are
best. The "long-haul" stations, therefore, work in the frequency range
immediately above 10,000 cycles a second and transmit with wave lengths
of 30,000 m. and shorter.

[Illustration: Pl. XII.--Broadcasting Station of the American Telephone
and Telegraph Company on the Roof of the Walker-Lispenard Bldg. in New
York City Where the Long-distance Telephone Lines Terminate.]

LETTER 24

BY WIRE AND BY RADIO

DEAR BOY:

The simplest wire telephone-circuit is formed by a transmitter, a
receiver, a battery, and the connecting wire. If two persons are to
carry on a conversation each must have this amount of equipment. The
apparatus might be arranged as in Fig. 129. This set-up, however,
requires four wires between the two stations and you know the telephone
company uses only two wires. Let us find the principle upon which its
system operates because it is the solution of many different problems
including that of wire-to-radio connections.

[Illustration: Fig 129]

Imagine four wire resistances connected together to form a square as in
Fig. 130. Suppose there are two pairs of equ