alf way up. You pull twice as hard but only
half as far and you do the same amount of work as before.

[Illustration: Fig 98]

We can't get more work out of the secondary of a transformer than we do
in the primary. If we design the transformer so that there is a greater
pull (e. m. f.) in the secondary the electron stream in the secondary
will be correspondingly smaller.

You remember how we measure resistance. We divide the e. m. f. (number
of volts) by the current (number of amperes) to find the resistance
(number of ohms). Suppose we do that for the primary and for the
secondary of the transformer of Fig. 98 which we are discussing. See
what happens in the secondary. There is only half as much voltage but
twice as much current. It looks as though the secondary had one-fourth
as much resistance as the primary. And so it has, but we usually call it
"impedance" instead of resistance because straight wires resist but
coils or condensers impede alternating e. m. f.'s.

[Illustration: Fig 99]

Before we return to the question of using a transformer in an audion
circuit let us turn this transformer around as in Fig. 99 and send the
current through the side with the larger number of windings. Let's talk
of "primary" and "secondary" just as before but, of course, remember
that now the primary has twice the turns of the secondary. On the
secondary side we shall have only half the current, but there will be
twice the e. m. f. The resistance of the secondary then is four times
that of the primary.

Now return to the amplifier of Fig. 94 and see what sort of a
transformer should be between the plate circuit of the tube and the
telephone receivers. Suppose the internal resistance of the tube is
12,000 ohms and the resistance of the telephones is 3,000 ohms. Suppose
also that the resistance (really impedance) of the primary side of the
transformer which we just considered is 12,000 ohms. The impedance of
its secondary will be a quarter of this or 3,000 ohms. If we connect
such a transformer in the circuit, as shown, we shall obtain the
greatest output from the tube.

In the first place the primary of the transformer has a number of ohms
just equal to the internal resistance of the tube. The tube, therefore,
will give its best to that transformer. In the second place the
secondary of the transformer has a resistance just equal to the
telephone receivers so it can give its best to them. The effect of the
transformer is to ma