at current any particular e. m. f. will cause depends upon the
circuit in which it is active. Circuits differ in the resistance which
they offer to e. m. f.'s. For any particular e. m. f. (that is for any
given e. m. f.) the resulting current will be smaller the greater the
resistance of the circuit. We measure resistance in ohms. To measure it
we find the quotient of the number of volts applied to the circuit by
the number of amperes which flow.

In my sixth letter I told you something of how the audion works. It
would be worth while to read again that letter. You remember that the
current in the plate circuit can be controlled by the e. m. f. which is
applied to the grid circuit. There is a relationship between the plate
current and the grid voltage which is peculiar or characteristic to the
tube. So we call such a relationship "a characteristic." Let us see how
it may be found and what it will be.

Connect an ammeter in the plate- or B-circuit, of the tube so as to
measure the plate-circuit current. You will find that almost all books
use the letter "_I_" to stand for current. The reason is that
scientists used to speak of the "intensity of an electric current" so
that "_I_" really stands for intensity. We use _I_ to stand
for something more than the word "current." It is our symbol for
whatever an ammeter would read, that is for the amount of current.

[Illustration: Fig 18]

Another convenience in symbols is this: We shall frequently want to
speak of the currents in several different circuits. It saves time to
use another letter along with the letter _I_ to show the circuit to
which we refer. For example, we are going to talk about the current in
the B-circuit of the audion, so we call that current _I_{B}_. We
write the letter _B_ below the line on which _I_ stands. That
is why we say the _B_ is subscript, meaning "written below." When
you are reading to yourself be sure to read _I_{B}_ as "eye-bee" or
else as "eye-subscript-bee." _I_{B}_ therefore will stand for the
number of amperes in the plate circuit of the audion. In the same way
_I_{a}_ would stand for the current in the filament circuit.

We are going to talk about e. m. f.'s also. The letter "_E_" stands
for the number of volts of e. m. f. in a circuit. In the filament
circuit the battery has _E_{A}_ volts. In the plate circuit the e.
m. f. is _E_{B}_ volts. If we put a battery in the grid circuit we
can let _E_{C}_ represent the number of volts appli