l which we call a "loading coil." That's why your friends add
a loading coil when they want to tune for the long wave-length stations,
that is, those with a low frequency.

When our circuit _L-C_ of Fig. 49 is tuned to the frequency of the
oscillator we get in it a maximum current. There is a maximum stream of
electrons, and hence a maximum number of them crowded first into one and
then into the other plate of the condenser. And so the condenser is
charged to a maximum voltage, first in one direction and then in the
other.

[Illustration: Fig 53]

Now connect the circuit _L-C_ to the grid of an audion. If the
circuit is tuned we'll have the maximum possible voltage applied between
grid and filament. In the plate circuit we'll get an increase and then a
decrease of current. You know that will happen for I prepared you for
this moment by the last page of my ninth letter. I'll tell you more
about that current in the plate circuit in a later letter. I am
connecting a telephone receiver in the plate circuit, and also a
condenser, the latter for a reason to be explained later. The
combination appears then as in Fig. 53. That figure shows a C-W
transmitter and an audion detector. This is the sort of a detector we
would use for radio-telephony, but the transmitter is the sort we would
use for radio-telegraphy. We shall make some changes in them later.

[Illustration: Fig 54]

Whenever we start the oscillating current in the transmitter we get an
effect in the detector circuit, of which I'll tell you more later. For
the moment I am interested in showing you how the transmitter and the
detector may be separated by miles and still there will be an effect in
the detector circuit every time the key in the transmitter circuit is
closed.

This is how we do it. At the sending station, that is, wherever we
locate the transmitter, we make a condenser using the earth, or ground,
as one plate. We do the same thing at the receiving station where the
detector circuit is located. To these condensers we connect inductances
and these inductances we couple to our transmitter and receiver as shown
in Fig. 54. The upper plate of the condenser in each case is a few
horizontal wires. The lower plate is the moist earth of the ground and
we arrange to get in contact with that in various ways. One of the
simplest methods is to connect to the water pipes of the city
water-system.

Now we have our radio transmitting-station and a station for rec