e the voltage by less
than that of a single battery cell. I want to show you a way which will.
You'll find it very useful to know and it is easily understood for it is
something like the arrangement of Fig. 14 in the preceding letter.

[Illustration: Fig 24]

Connect the cells as in Fig. 24 to a fine wire. About the middle of this
wire connect the filament. As before use a clip on the end of the wire
from the grid. If the grid is connected to _a_ in the figure there
is applied to the grid circuit that part of the e. m. f. of the battery
which is active in the length of wire between _o_ and _a_. The
point _a_ is nearer the positive plate of the battery than is the
point _o_. So the grid will be positive and the filament negative.

On the other hand, if the clip is connected at _b_ the grid will be
negative with respect to the filament. We can, therefore, make the grid
positive or negative depending on which side of _o_ we connect the
clip. How large the e. m. f. is which will be applied to the grid
depends, of course, upon how far away from _o_ the clip is
connected.

Suppose you took the clip in your hand and slid it along in contact with
the wire, first from _o_ to _a_ and then back again through
_o_ to _b_ and so on back and forth. You would be making the
grid _alternately_ positive and negative, wouldn't you? That is,
you would be applying to the grid an e. m. f. which increases to some
positive value and then, decreasing to zero, _reverses_, and
increases just as much, only to decrease to zero, where it started. If
you do this over and over again, taking always the same time for one
round trip of the clip you will be impressing on the grid circuit an
"_alternating e. m. f._"

What's going to happen in the plate circuit? When there is no e. m. f.
applied to the grid circuit, that is when the grid potential
(possibilities) is zero, there is a definite current in the plate
circuit. That current we can find from our characteristic of Fig. 23 for
it is where the curve crosses Zero Volts. As the grid becomes positive
the current rises above this value. When the grid is made negative the
current falls below this value. The current, _I_{B}_, then is made
alternately greater and less than the current when _E_{C}_ is zero.

You might spend a little time thinking over this, seeing what happens
when an alternating e. m. f. is applied to the grid of an audion, for
that is going to be fundamental to our study of radio.