We call the resistance which the
tube offers to the passage of electrons between _P_ and _F_
the "internal resistance" of the plate circuit of the tube. How large it
is depends upon the design of tube. In some tubes it may be five or six
thousand ohms, and in others several times as high. In the large tubes
used in high-powered transmitting sets it is much less. Since it will be
different in different cases we shall use a symbol for it and say that
it is _R_{p}_ ohms.

Then one rule for using an audion as an amplifier is this: To get the
most out of an audion see that the telephone, or whatever circuit or
piece of apparatus is connected to the output terminals, shall have a
resistance of _R_{p}_ ohms. When the resistance of the circuit,
which an audion is supplying with current, is the same as the internal
resistance of the output side of the tube, then the audion gives its
greatest output. That is the condition for the greatest "amount of
energy each second," or the "greatest power" as we say.

That rule is why we always select the telephone receivers which we use
with an audion and always ask carefully as to their resistance when we
buy. Sometimes, however, it is not practicable to use receivers of just
the right resistance. Where we connect the output side of an audion to
some other circuit, as where we let one audion supply another, it is
usually impossible to follow this rule without adding some special
apparatus.

This leads to the next rule: If the telephone receiver, or the circuit,
which we wish to connect to the output of an audion, does not have quite
nearly a resistance of _R_{p}_ ohms we use a properly designed
transformer as we have already done in Figs. 94 and 95.

A transformer is two separate coils coupled together so that an
alternating current in the primary will induce an alternating current in
the secondary. Of course, if the secondary is open-circuited then no
current can flow but there will be induced in it an e. m. f. which is
ready to act if the circuit is closed. Transformers have an interesting
ability to make a large resistance look small or vice versa. To show you
why, I shall have to develop some rules for transformers.

Suppose you have an alternating e. m. f. of ten volts applied to the
primary of an iron-cored transformer which has ten turns. There is one
volt applied to each turn. Now, suppose the secondary has only one turn.
That one turn has induced in it an alternating e. m. f.