ns
in current which may naturally occur in the action of the batteries.
There are always small variations in the currents from batteries, due to
impurities in the materials of the plates, air bubbles, and other
causes. Ordinarily we don't observe these changes because they are too
small to make an audible sound in the telephone receivers. Suppose,
however, that they take place in the battery of the first tube of a
series of amplifiers. Any tiny change of current is amplified many times
and results in a troublesome noise in the telephone receiver which is
connected to the last tube.

In both types of amplifiers there is, of course, always the chance that
the output circuit of one tube may be coupled to and induce some effect
in the input circuit of one of the earlier tubes of the series. This
will be amplified and result in a greater induction. In other words, in
a circuit where there is large amplification, there is always the
difficulty of avoiding a feed-back of energy from one tube to another so
that the entire group acts like an oscillating circuit, that is
"regeneratively." Much of this difficulty can be avoided after
experience.

If a multi-stage amplifier is to be built for a current which does not
have too high a frequency the "capacity effects" and the other
difficulties due to high-frequency need not be seriously troublesome. If
the frequency is not too high, but is still well above the audible
limit, the noises due to variations in battery currents need not bother
for they are of quite low frequency. Currents from 20,000 to 60,000
cycles a second are, therefore, the most satisfactory to amplify.

Suppose, however, one wishes to amplify the signals from a
radio-broadcasting station. The wave-length is 360 meters and the
frequency is about 834,000 cycles a second. The system of
intermediate-frequency amplification solves the difficulty and
we shall see how it does so.

[Illustration: Fig 128]

At the receiving station a local oscillator is used. This generates a
frequency which is about 30,000 cycles less than that of the incoming
signal. Both currents are impressed on the grid of a detector. The
result is, in the output of the detector, a current which has a
frequency of 30,000 cycles a second. The intensity of this detected
current depends upon the intensity of the incoming signal. The "beat
note" current of 30,000 cycles varies, therefore, in accordance with the
voice which is modulating at the distan