ced current is in
just the right direction to send more electrons into waiting-room 1 and
so to make the grid still more negative. And the more negative the grid
gets the smaller becomes the plate current until finally the plate
current is reduced to zero. Look at the audion characteristic again and
see that making the grid sufficiently negative entirely stops the plate
current.

When the plate current stops, the condenser in the grid circuit is
charged, with plate 1 negative and 2 positive. It was the plate current
which was the main cause of this change for it induced the charging
current in coil _cd_. So, when the plate current becomes zero there
is nothing to prevent the condenser from discharging.

Its discharge makes the grid less and less negative until it is zero
volts and there we are--back practically where we started. The plate
current is increasing and the grid is getting positive, and we're off on
another "cycle" as we say. During a cycle the plate current increases to
a maximum, decreases to zero, and then increases again to its initial
value.

[Illustration: Fig 36]

This letter has a longer continuous train of thought than I usually ask
you to follow. But before I stop I want to give you some idea of what
good this is in radio.

What about the current which flows in coil _cd_? It's an
alternating current, isn't it? First the electrons stream from _d_
towards _c_, and then back again from _c_ towards _d_.

Suppose we set up another coil like _CD_ in Fig. 36. It would have
an alternating current induced in it. If this coil was connected to an
antenna there would be radio waves sent out. The switch _S_ could
be used for a key and kept closed longer or shorter intervals depending
upon whether dashes or dots were being set. I'll tell you more about
this later, but in this diagram are the makings of a "C-W Transmitter,"
that is a "continuous wave transmitter" for radio-telegraphy.

It would be worth while to go over this letter again using a pencil and
tracing in the various circuits the electron streams which I have
described.

LETTER 12

INDUCTANCE AND CAPACITY

DEAR SIR:

In the last letter I didn't stop to draw you a picture of the action of
the audion oscillator which I described. I am going to do it now and you
are to imagine me as using two pencils and drawing simultaneously two
curves. One curve shows what happens to the current in the plate
circuit. The other shows how the volt