e impedance to act like _L_{a}_ and so it
permits the modulator to vary the output of the oscillator.

[Illustration: Fig 126]

The oscillating circuit of Fig. 125 includes part of the antenna. It
differs also from the others I have shown in the manner in which grid
and plate circuits are coupled. I'll explain by Fig. 126.

The transmitting set which I have just described involves many of the
principles of the most modern sets. If you understand its operation you
can probably reason out for yourself any of the other sets of which you
will hear from time to time.

LETTER 23

AMPLIFICATION AT INTERMEDIATE FREQUENCIES

DEAR SON:

In the matter of receiving I have already covered all the important
principles. There is one more system, however, which you will need to
know. This is spoken of either as the "super-heterodyne" or as the
"intermediate-frequency amplification" method of reception.

The system has two important advantages. First, it permits sharper
tuning and so reduces interference from other radio signals. Second, it
permits more amplification of the incoming signal than is usually
practicable.

First as to amplification: We have seen that amplification can be
accomplished either by amplifying the radio-frequency current before
detection or by amplifying the audio-frequency current which results
from detection. There are practical limitations to the amount of
amplification which can be obtained in either case. An efficient
multi-stage amplifier for radio-frequencies is difficult to build
because of what we call "capacity effects."

Consider for example the portion of circuit shown in Fig. 127. The wires
_a_ and _b_ act like small plates of condensers. What we
really have, is a lot of tiny condensers which I have shown in the
figure by the light dotted-lines. If the wires are transmitting
high-frequency currents these condensers offer tiny waiting-rooms where
the electrons can run in and out without having to go on to the grid of
the next tube. There are other difficulties in high-frequency
amplifiers. This one of capacity effects between parallel wires is
enough for the present. It is perhaps the most interesting because it is
always more or less troublesome whenever a pair of wires is used to
transmit an alternating current.

[Illustration: Fig 127]

In the case of a multi-stage amplifier of audio-frequency current there
is always the possibility of the amplification of any small variatio