OTHER ELECTROMAGNETIC DEVICES

DEAR SON:

In an earlier letter when we first introduced a telephone receiver into
a circuit I told you something of how it operates. I want now to tell
why and also of some other important devices which operate for the same
reason.

You remember that a stream of electrons which is starting or stopping
can induce the electrons of a neighboring parallel circuit to start off
in parallel paths. We do not know the explanation of this. Nor do we
know the explanation of another fact which seems to be related to this
fact of induction and is the basis for our explanations of magnetism.

[Illustration: Fig 101]

If two parallel wires are carrying steady electron streams in the same
general direction the wires attract each other. If the streams are
oppositely directed the wires repel each other. Fig. 101 illustrates
this fact. If the streams are not at all in the same direction, that is,
if they are at right angles, they have no effect on each other.

[Illustration: Fig 102]

These facts, of the attraction of electron streams which are in the same
direction and repulsion of streams in opposite directions, are all that
one need remember to figure out for himself what will happen under
various conditions. For example, if two coils of wire are carrying
currents what will happen is easily seen. Fig. 102 shows the two coils
and a section through them.

[Illustration: Fig 103]

Looking at this cross section we seem to have four wires, _1_ and
_2_ of coil _A_ and _3_ and _4_ of coil _B_. You see at once that
if the coils are free to move they will move into the dotted positions
shown in Fig 102, because wire _1_ attracts wire _3_ and repels wire
_4_, while wire _2_ attracts wire _4_ and repels wire _3_. If
necessary, and if they are free to move, the coils will turn
completely around to get to this position. I have shown such a case
in Fig. 103.

Wires which are not carrying currents do not behave in this way. The
action is due, but how we don't yet know, to the motions of the
electrons. As far as we can explain it to-day, the attraction of two
wires which are carrying currents is due to the attraction of the two
streams of electrons. Of course these electrons are part of the wires.
They can't get far away from the stay-at-home electrons and the nuclei
of the atoms which form the wires. In fact it is these nuclei which keep
the wandering electrons within the wires. The result is that if the