al Radio Co.]

And like the loops of Fig. 106 the atoms in an unmagnetized piece of
iron are pretty well satisfied to stay as they are without all lining up
to pull together. To magnetize the iron we must force some of these
atomic loops to turn part way around. That can be done by bringing near
them a strong magnet or a coil of wire which is carrying a current. Then
the atoms are forced to turn and if enough turn so that there is an
appreciable effect then the iron is magnetized. The more that are
properly turned the stronger is the magnet. One end or "pole" we call
north-seeking and the other south-seeking, because a magnetized bar of
iron acts like a compass needle.

[Illustration: Fig 107]

A coil of wire, carrying a current, acts just like a magnet because its
larger loops are all ready to pull together. I have marked the coil of
Fig. 107 with _N_ and _S_ for north and south. If the electron
stream in it is reversed the "polarity" is reversed. There is a simple
rule for this. Partially close your left hand so that the fingers form
loops. Let the thumb stick out at right angles to these loops. If the
electron streams are flowing around the loops of a coil in the same
direction as your fingers point then your thumb is the _N_ pole and
the coil will repel the north poles of other loops or magnets in the
direction in which your thumb points. If you know the polarity already
there is a simple rule for the repulsion or attraction. Like poles
repel, unlike poles attract.

From what has been said about magnetism you can now understand why in a
telephone receiver the current in the winding can make the magnet
stronger. It does so because it makes more of the atomic loops of the
iron turn around and help pull. On the other hand if the current in the
winding is reversed it will turn some of the loops which are already
helping into other positions where they don't help and may hinder. If
the current in the coil is to help, the electron stream in it must be so
directed that the north pole of the coil is at the same end as the north
pole of the magnet.

This idea of the attraction or repulsion of electron streams, whether in
coils of wire or in atoms of iron and other magnetizable substances, is
the fundamental idea of most forms of telephone receivers, of electric
motors, and of a lot of other devices which we call "electromagnetic."

The ammeters and voltmeters which we use for the measurement of audion
characteristics