er wire very easily. So if we fasten one end
of the copper wire to the carbon and the other end to the zinc, the
electrons will flow from the zinc to the carbon as long as there are
more electrons on the zinc; that is, until the battery wears out.
Therefore we have a steady flow of electricity through the wire. While
the electricity is flowing from one pole to the other, we can make it
do work.

EXPERIMENT 64. Set up two or three Samson cells. They consist
of a glass jar, an open zinc cylinder, and a smaller carbon
cylinder. Dissolve a little over half a cup of sal ammoniac
in water and put it into the glass jar; then fill the jar with
water up to the line that is marked on it. Put the carbon and
zinc which are attached to the black jar cover into the jar.
Be careful not to let the carbon touch the zinc. One of these
cells will probably not be strong enough to ring a doorbell
for you; so connect two or three together in series as
follows:

Fasten a piece of copper wire from the carbon of the first
cell to the zinc of the second. If you have three cells,
fasten another piece of wire from the carbon of the second
cell to the zinc of the third, as shown in Figure 111.

Fasten one end of a copper wire to the zinc of the first
cell and the other end of this wire to one binding post of an
electric bell. Fasten one end of another piece of copper wire
to the carbon of the third cell, if you have three, and touch
the other end of this wire to the free binding post of the
electric bell. If you have everything connected rightly, the
bell should ring.

[Illustration: FIG. 111. A wet battery of three cells connected to ring
a bell.]

DIFFERENT KINDS OF BATTERIES. There are many different kinds of
batteries. The one you have just made is a simple one frequently used
for doorbells. Other batteries are more complicated. Some are made
with copper and zinc in a solution of copper sulfate; some, even, are
made by letting electricity from a dynamo run through a solution from
one lead plate to another until a chemical substance is stored on one
of them; then, when the two lead plates are connected by a wire, the
electrons run from one to the other. This kind of battery is called a
_storage battery_, and it is much used in submarines and automobiles.

[Illustration: FIG. 112. A battery of three dry cells.]

But all the different batteries work on th