e same general principle: A
chemical solution divides into two parts, one with many electrons and
the other with a less number. One part of the solution gathers on one
pole (piece of metal in the solution) and charges it positively; the
other part gathers on the other pole and charges it negatively. Then
the electricity flows from one pole to the other.

POSITIVE AND NEGATIVE POLES. Before people knew anything about
electrons, they knew that electricity flowed from one pole of a
battery to the other. But they always said that it flowed from the
carbon to the zinc; and they called the carbon the positive pole and
the zinc the negative. Although we now know that the electrons flow
from the zinc to the carbon, it is much more convenient to use the old
way of speaking, as was explained on page 199. Practically, it makes
no difference which way the electrons are going as long as a current
of electricity is flowing through the wire from one pole of the
battery to the other pole. So every one speaks of electricity as
flowing from the positive pole of a battery (usually the carbon
or copper) to the negative pole (usually the zinc), although the
electrons actually move in the other direction.

[Illustration: FIG. 113. A storage battery.]

Batteries make enough electricity flow to do a good deal of work. But
they are rather expensive, and it takes a great many to give a flow of
electricity sufficient for really heavy work, such as running street
cars or lighting a city. Fortunately there is another way of getting
large amounts of electricity to flow. This is by means of dynamos.

HOW A DYNAMO MAKES A CURRENT FLOW. To understand a dynamo, you must
first realize that there are countless electrons in the world--perhaps
all things are made entirely of them. But you remember that when we
want to get these electrons to do work we must make them flow. This
can be done by spinning a loop of wire between the poles of a magnet.
Whenever a loop of wire is turned between the two poles of a magnet,
the magnetism pushes the electrons that are already in the wire around
and around the loop. As long as we keep the loop spinning, a current
of electricity flows.

[Illustration: FIG. 114. Spinning loops of wire between the poles of a
magnet causes a current of electricity to flow through the wire.]

If only one loop of wire is spun between the poles of a magnet, the
current is very feeble. If you loop the wire around twice, as shown in
Figur