ed to the
grid-filament or C-circuit.

The characteristic relation which we are after is one between grid
voltage, that is _E_{C}_, and plate current, that is _I_{B}_.
So we call it the _E_{C}_--_I_{B}_ characteristic. The dash
between the letters is not a subtraction sign but merely a dash to
separate the letters. Now we'll find the "ee-see-eye-bee"
characteristic.

Connect some small dry cells in series for use in the grid circuit. Then
connect the filament to the middle cell as in Fig. 19. Take the wire
which comes from the grid and put a battery clip on it, then you can
connect the grid anywhere you want along this series of batteries. See
Fig. 18. In the figure this movable clip is represented by an arrow
head. You can see that if it is at _a_ the battery will make the
grid positive. If it is moved to _b_ the grid will be more
positive. On the other hand if the clip is at _o_ there will be no
e. m. f. applied to the grid. If it is at _c_ the grid will be made
negative.

Between grid and filament there is placed a voltmeter which will tell
how much e. m. f. is applied to the grid, that is, tell the value of
_E_{C}_, for any position whatever of the clip.

We shall start with the filament heated to a deep red. The manufacturers
of the audion tell the purchaser what current should flow through the
filament so that there will be the proper emission of electrons. There
are easy ways of finding out for one's self but we shall not stop to
describe them. The makers also tell how many volts to apply to the
plate, that is what value _E_{B}_ should have. We could find this
out also for ourselves but we shall not stop to do so.

[Illustration: Fig 19]

Now we set the battery clip so that there is no voltage applied to the
grid; that is, we start with _E_{C}_ equal to zero. Then we read the
ammeter in the plate circuit to find the value of _I_{B}_ which
corresponds to this condition of the grid.

Next we move the clip so as to make the grid as positive as one battery
will make it, that is we move the clip to _a_ in Fig. 19. We now
have a different value of _E_{C}_ and will find a different value
of _I_{B}_ when we read the ammeter. Next move the clip to apply
two batteries to the grid. We get a new pair of values for _E_{C}_
and _I_{B}_, getting _E_{C}_ from the voltmeter and _I_{B}_ from the
ammeter. As we continue in this way, increasing _E_{C}_, we find that
the current _I_{B}_ increases for a while and then after