through the telephone receiver run
into one plate of the condenser. Of course, at that same instant an
equal number leave the other plate and start off toward the B-battery
and the filament. An instant later, when the current in the plate
circuit is small, electrons start to come out of the plate and to join
the stream through the receiver so that this stream is kept steady.

[Illustration: Fig 63]

This steady stream of electrons, which is passing through the receiver
winding, is larger than it would be if there was no incoming radio
signal. The result is a stronger pull on the diaphragm of the receiver.
The moment the signal starts this diaphragm is pulled over toward the
magnet and it stays pulled over as long as the signal lasts. When the
signal ceases it flies back. We would hear then a click when the signal
started and another when it stopped.

If we wanted to distinguish dots from dashes this wouldn't be at all
satisfactory. So in the next letter I'll show you what sort of changes
we can make in the apparatus. To understand what effect these changes
will have you need, however, to understand pretty well most of this
letter.

LETTER 15

RADIO-TELEPHONY

DEAR LAD:

Before we start on the important subject matter of this letter let us
make a short review of the preceding two letters.

An oscillating audion at the transmitting station produces an effect on
the plate current of the detector audion at the receiving station. There
is impressed upon the grid of the detector an alternating e. m. f. which
has the same frequency as the alternating current which is being
produced at the sending station. While this e. m. f. is active, and of
course it is active only while the sending key is held down, there is
more current through the winding of the telephone receiver and its
diaphragm is consequently pulled closer to its magnet.

What will happen if the e. m. f. which is active on the grid of the
detector is made stronger or weaker? The pull on the receiver diaphragm
will be stronger or weaker and the diaphragm will have to move
accordingly. If the pull is weaker the elasticity of the iron will move
the diaphragm away from the magnet, but if the pull is stronger the
diaphragm will be moved toward the magnet.

Every time the diaphragm moves it affects the air in the immediate
neighborhood of itself and that air in turn affects the air farther away
and so the ear of the listener. Therefore if there are cha