separate circuits. One between _G_ and _F_ appears
to be an open circuit. The other appears to have in it a resistance and
a battery (or else an alternator). The e. m. f. of the battery, or
alternator, as the case may be, depends on what source of e. m. f. is
connected to _G-F_. Whatever that e. m. f. is, there is a
corresponding kind of e. m. f. inside the box but one several times
larger.

[Illustration: Fig 97]

We might, therefore, pay no further attention to what is actually inside
the box or how all these effects are brought about. We might treat the
entire box as if it was formed by two separate circuits as shown in Fig.
97. If we do so, we are replacing the box by something which is
equivalent so far as effects are concerned, that is we are replacing an
actual audion by two circuits which together are equivalent to it.

The men who first performed such experiments wanted some convenient way
of saying that if an alternator, which has an e. m. f. of _V_
volts, is connected to _F_ and _G_, the effect is the same as
if a much stronger alternator is connected between _F_ and
_P_. How much stronger this imaginary alternator is depends upon
the design of the audion. For some audions it might be five times as
strong, for other designs 6.5 or almost any other number, although
usually a number of times less than 40. They used a little Greek letter
called "mu" to stand for this number which depends on the design of the
tube. Then they said that the hidden alternator in the output circuit
was mu times as strong as the actual alternator which was applied
between the grid and the filament. Of course, instead of writing the
sound and name of the letter they used the letter [Greek: m] itself. And
that is what I have done in the sketch of Fig. 97.

Now we are ready to talk about the audion as an amplifier. The first
thing to notice is the fact that we have an open circuit between
_F_ and _G_. This is true as long as we don't apply an e. m.
f. large enough to overcome the C-battery of Fig. 96 and thus let the
grid become positive and attract electrons from the filament. We need
then spend no further time thinking about what will happen in the
circuit _G-F_, for there will be no current.

As to the circuit _F-P_, we can treat it as a resistance in series
with which there is a generator [Greek: m] times as strong as that which
is connected to _F_ and _G_. The next problem is how to get
the most out of this hidden generator.