g in the plate circuit. That changes the plate current. The next
step is to shift the slider in the grid circuit until we have again the
original value of current in the plate circuit. Suppose that the tube is
ordinarily run with a plate voltage of 40 volts and we start with that
e. m. f. on the plate. Suppose that we now make it 50 volts and then
vary the position of the slider in the grid circuit until the ammeter
reads as it did at the start. Next we read the voltage impressed on the
grid by reading the voltmeter in the grid circuit. Suppose it reads 2
volts. What does that mean?

[Illustration: Fig 94]

It means that two volts in the grid circuit have the same effect on the
plate current as ten volts in the plate circuit. If we apply a volt to
the grid circuit we get five times as large an effect in the plate
circuit as we would if the volt were applied there. We get a greater
effect, the effect of more volts, by applying our voltage to the grid.
We say that the tube acts as an "amplifier of voltage" because we can
get a larger effect than the number of volts which we apply would
ordinarily entitle us to.

Now let's take a simple case of the use of an audion as an amplifier.
Suppose we have a receiving circuit with which we find that the signals
are not easily understood because they are too weak. Let this be the
receiving circuit of Fig. 88 which I am reproducing here as part of Fig.
94.

We have replaced the telephone receiver by a "transformer." A
transformer is two coils, or windings, coupled together. An alternating
current in one will give rise to an alternating current in the other.
You are already familiar with the idea but this is our first use of the
word. Usually we call the first coil, that is the one through which the
alternating current flows, the "primary" and the second coil, in which a
current is induced, the "secondary."

The secondary of this transformer is connected to the grid circuit of
another vacuum tube, to the plate circuit of which is connected another
transformer and the telephone receiver. The result is a detector and
"one stage of amplification."

The primary of the first transformer, so we shall suppose, has in it the
same current as would have been in the telephone. This alternating
current induces in the secondary an e. m. f. which has the same
variations as this current. This e. m. f. acts on the grid of the second
tube, that is on the amplifier. Because the audion amplifies,