harder than
through a straight wire, but after they are going they don't like to
stop. They like it much less if they are going through a coil instead of
a straight wire.

In my next letter I'll tell you what happens when we have a coil and a
condenser together in a circuit.

[Footnote 4: The "henry" has nothing to do with a well-known automobile.
It was named after Joseph Henry, a professor years ago at Princeton
University.]

LETTER 11

A "C-W" TRANSMITTER

DEAR SON:

[Illustration: Fig 28]

Let's look again at the coils of Fig. 28 which we studied in the last
letter. I have reproduced them here so you won't have to turn back. When
electrons start from _a_ towards _b_ there is a momentary
stream of electrons from _d_ towards _c_. If the electron
stream through _ab_ were started in the opposite direction, that is
from _b_ to _a_ the induced stream in the coil _cd_ would
be from _c_ towards _d_.

[Illustration: Fig 30]

It all reminds me of two boys with a hedge or fence between them as in
Fig. 30. One boy is after the other. Suppose you were being chased; you
know what you'd do. If your pursuer started off with a rush towards one
end of the hedge you'd "beat it" towards the other. But if he started
slowly and cautiously you would start slowly too. You always go in the
opposite direction, dodging back and forth along the paths which you are
wearing in the grass on opposite sides of the hedge. If he starts to the
right and then slows up and starts back, you will start to your right,
slow up, and start back. Suppose he starts at the center of the hedge.
First he dodges to the right, and then back through the center as far to
the left, then back again and so on. You follow his every change.

[Illustration: Fig 31]

I am going to make a picture of what you two do. Let's start with the
other fellow. He dodges or alternates back and forth. Some persons would
say he "oscillates" back and forth in the same path. As he does so he
induces you to move. I am on your side of the hedge with a
moving-picture camera. My camera catches both of you. Fig. 31 shows the
way the film would look if it caught only your heads. The white circle
represents the tow-head on my side of the hedge and the black circle,
young Brown who lives next door. Of course, the camera only catches you
each time the shutter opens but it is easy to draw a complete picture of
what takes place as time goes on. See Fig. 32.

[Illustration: F