ire you hold in your hands passes the iron face of the magnet,
a wave of energy that is called electricity flows around this loop at
the rate of 186,000 miles a second--the same speed as light comes to
us from the sun. As you move the wire away from the magnet, a second
wave starts through the wire, flowing in the opposite direction. You
can prove this by holding a compass needle under the wire and see it
wag first in one direction, then in another.

[Illustration: A wire "cutting" the lines of force of an
electro-magnet]

This is a simple dynamo. A wire "cutting" the invisible lines of
force, that a magnet is spraying out into the air, becomes
"electrified." Why this is true, no one has ever been able to explain.

The amount of electricity--its capacity for work--which you have
generated with the magnet and wire, does not depend alone on the
pulling power of that simple magnet. Let us say the magnet is very
weak--has not enough power to lift one ounce of iron. Nevertheless,
if you possessed the strength of Hercules, and could pass that wire
through the field of force of the magnet many thousands of times a
second, you would generate enough electricity in the wire to cause the
wire to melt in your hands from heat.

[Illustration: Cross-section of an armature revolving in its field]

[Illustration: Forms of annealed steel discs used in armature
construction]

This experiment gives the theory of the dynamo. Instead of passing
only one wire through the field of force of a magnet, we have hundreds
bound lengthwise on a revolving drum called an armature. Instead of
one magnetic pole in a dynamo we have two, or four, or twenty
according to the work the machine is designed for--always in pairs, a
North pole next to a South pole, so that the lines of force may flow
out of one and into another, instead of escaping in the surrounding
air. If you could see these lines of force, they would appear in
countless numbers issuing from each pole face of the field magnets,
pressing against the revolving drum like hair brush bristles--trying
to hold it back. This drum, in practice, is built up of discs of
annealed steel, and the wires extending lengthwise on its face are
held in place by slots to prevent them from flying off when the drum
is whirled at high speed. The drum does not touch the face of the
magnets, but revolves in an air space. If we give the electric
impulses generated in these wires a chance to flow in a circuit--flo