of one volt. If
there are more turns of wire forming the secondary, then each turn has
induced in it just one volt. But the e. m. f.'s of all these turns add
together. If the secondary has twenty turns, there is induced in it a
total of twenty volts. So the first rule is this: In a transformer the
number of volts in each turn of wire is just the same in the secondary
as in the primary.

If we want a high-voltage alternating e. m. f. all we have to do is to
send an alternating current through the primary of a transformer which
has in the secondary, many times more turns of wire than it has in the
primary. From the secondary we obtain a higher voltage than we impress
on the primary.

You can see one application of this rule at once. When we use an audion
as an amplifier of an alternating current we send the current which is
to be amplified through the primary of a transformer, as in Fig. 94. We
use a transformer with many times more turns on the secondary than on
the primary so as to apply a large e. m. f. to the grid of the
amplifying tube. That will mean a large effect in the plate circuit of
the amplifier.

You remember that the grid circuit of an audion with a proper value of
negative C-battery is really open-circuited and no current will flow in
it. For that case we get a real gain by using a "step-up" transformer,
that is, one with more turns in the secondary than in the primary.

It looks at first as if a transformer would always give a gain. _If we
mean a gain in energy it will not_ although we may use it, as we
shall see in a minute, to permit a vacuum tube to work into an output
circuit more efficiently than it could without the transformer. We
cannot have any more energy in the secondary circuit of a transformer
than we give to the primary.

Suppose we have a transformer with twice as many turns on the secondary
as on the primary. To the primary we apply an alternating e. m. f. of a
certain number of volts. In the secondary there will be twice as many
volts because it has twice as many turns. The current in the secondary,
however, will be only half as large as is the current in the primary. We
have twice the force in the secondary but only half the electron stream.

It is something like this: You are out coasting and two youngsters ask
you to pull them and their sleds up hill. You pull one of them all the
way and do a certain amount of work. On the other hand suppose you pull
them both at once but only h