mes.

_Magnets and Magnetism_

The dynamo is a mechanical engine, like the steam engine, the water
turbine or the gas engine; and it converts the mechanical motion of
the driven wheel into electrical motion, with the aid of a magnet.
Many scientists say that the full circle of energy that keeps the
world spinning, grows crops, and paints the sky with the Aurora
Borealis, begins and ends with magnetism--that the sun's rays are
magnetic rays. Magnetism is the force that keeps the compass needle
pointing north and south. Take a steel rod and hold it along the
north and south line, slightly inclined towards the earth, and strike
it a sharp blow with a hammer, and it becomes a magnet--feeble, it is
true, but still a magnet.

Take a wire connected with a common dry battery and hold a compass
needle under it and the needle will immediately turn around and point
directly across the wire, showing that the wire possesses magnetism
encircling it in invisible lines, stronger than the magnetism of the
earth.

[Illustration: (_Courtesy of the Crocker-Wheeler Company_)

A direct-current dynamo or motor, showing details of construction]

Insulate this wire by covering it with cotton thread, and wind it
closely on a spool. Connect the two loose ends to a dry battery, and
you will find that you have multiplied the magnetic strength of a
single loop of wire by the number of turns on the spool--concentrated
all the magnetism of the length of that wire into a small space. Put
an iron core in the middle of this spool and the magnet seems still
more powerful. Lines of force which otherwise would escape in great
circles into space, are now concentrated in the iron. The iron core
is a magnet. Shut off the current from the battery and the iron is
still a magnet--weak, true, but it will always retain a small portion
of its magnetism. Soft iron retains very little of its magnetism. Hard
steel retains a great deal, and for this reason steel is used for
permanent magnets, of the horseshoe type so familiar.

_A Simple Dynamo_

A dynamo consists, first, of a number of such magnets, wound with
insulated wire. Their iron cores point towards the center of a circle
like the spokes of a wheel; and their curved inner faces form a circle
in which a spool, wound with wire in another way, may be spun by the
water wheel.

Now take a piece of copper wire and make a loop of it. Pass one side
of this loop in front of an electric magnet.

As the w