he same voltage as the
condenser to which it is connected and so it becomes alternately
positive and negative. This state of affairs starts almost as soon as
the key at the sending station is depressed and continues as long as it
is held down.

Now what happens inside the audion? As the grid becomes more and more
positive the current in the plate circuit increases. When the grid no
longer grows more positive but rather becomes less and less positive the
current in the plate circuit decreases. As the grid becomes of zero
voltage and then negative, that is as the grid "reverses its polarity,"
the plate current continues to decrease. When the grid stops growing
more negative and starts to become less so, the plate current stops
decreasing and starts to increase.

All this you know, for you have followed through such a cycle of changes
before. You know also how we can use the audion characteristic to tell
us what sort of changes take place in the plate current when the grid
voltage changes. The plate current increases and decreases alternately,
becoming greater and less than it would be if the grid were not
interfering. These variations in its intensity take place very rapidly,
that is with whatever high frequency the sending station operates. What
happens to the plate current on the average?

The plate current, you remember, is a stream of electrons from the
filament to the plate (on the inside of the tube), and from the plate
back through the B-battery to the filament (on the outside of the tube).
The grid alternately assists and opposes that stream. When it assists,
the electrons in the plate circuit are moved at a faster rate. When the
grid becomes negative and opposes the plate the stream of electrons is
at a slower rate. The stream is always going in the same direction but
it varies in its rate depending upon the changes in grid potential.

[Illustration: Fig 55]

When the grid is positive, that is for half a cycle of the alternating
grid-voltage, the stream is larger than it would be if the plate current
depended only on the B-battery. For the other half of a cycle it is
less. The question I am raising is this: Do more electrons move around
the plate circuit if there is a signal coming in than when there is no
incoming signal? To answer this we must look at the audion
characteristic of our particular tube and this characteristic must have
been taken with the same B-battery as we use when we try to receive the
s