the e. m.
f. acting on the telephone receiver is larger than it would have been
without the use of this audion. And hence there is a greater response on
the part of its diaphragm and a louder sound.

In setting up such a circuit as this there are several things to watch.
For some of these you will have to rely on the dealer from whom you buy
your supplies and for the others upon yourself. But it will take another
letter to tell you of the proper precautions in using an audion as an
amplifier.

[Illustration: Fig 95]

In the circuit which I have just described an audion is used to amplify
the current which comes from the detector before it reaches the
telephone receiver. Sometimes we use an audion to amplify the e. m. f.
of the signal before impressing it upon the grid of the detector. Fig.
95 shows a circuit for doing that. In the case of Fig. 94 we are
amplifying the audio-frequency current. In that of Fig. 95 it is the
radio-frequency effect which is amplified. The feed-back or regenerative
circuit of Fig. 92 is a one-tube circuit for doing the same thing as is
done with two tubes in Fig 95.

[Footnote 9: There is always some amplification taking place in an
audion detector but the regenerative circuit amplifies over and over
again until the signal is as large as the tube can detect.]

LETTER 19

THE AUDION AMPLIFIER AND ITS CONNECTIONS

DEAR SON:

In our use of the audion we form three circuits. The first or A-circuit
includes the filament. The B-circuit includes the part of the tube
between filament and plate. The C-circuit includes the part between
filament and grid. We sometimes speak of the C-circuit as the "input"
circuit and the B-circuit as the "output" circuit of the tube. This is
because we can put into the grid-filament terminals an e. m. f. and
obtain from the plate-filament circuit an effect in the form of a change
of current.

[Illustration: Fig 96]

Suppose we had concealed in a box the audion and circuit of Fig. 96 and
that only the terminals which are shown came through the box. We are
given a battery and an ammeter and asked to find out all we can as to
what is between the terminals _F_ and _G_. We connect the
battery and ammeter in series with these terminals. No current flows
through the circuit. We reverse the battery but no current flows in the
opposite direction. Then we reason that there is an open-circuit between
_F_ and _G_.

As long as we do not use a higher voltage