at the copper of one cell is
connected with the zinc of another cell, the electromotive force of
the battery will be three times as great as the E.M.F. of a single
cell. If four cells are arranged in the same way, the E.M.F. of the
battery is four times as great as the E.M.F. of a single cell; when
five cells are combined, the resulting E.M.F. is five times as great.

CHAPTER XXXI

SOME USES OF ELECTRICITY

288. Heat. Any one who handles electric wires knows that they are
more or less heated by the currents which flow through them. If three
cells are arranged as in Figure 200 and the connecting wire is coarse,
the heating of the wire is scarcely noticeable; but if a shorter wire
of the same kind is used, the heat produced is slightly greater; and
if the coarse wire is replaced by a short, fine wire, the heating of
the wire becomes very marked. We are accustomed to say that a wire
offers resistance to the flow of a current; that is, whenever a
current meets resistance, heat is produced in much the same way as
when mechanical motion meets an obstacle and spends its energy in
friction. The flow of electricity along a wire can be compared to the
flow of water through pipes: a small pipe offers a greater resistance
to the flow of water than a large pipe; less water can be forced
through a small pipe than through a large pipe, but the friction of
the water against the sides of the small pipe is much greater than in
the large one.

So it is with the electric current. In fine wires the resistance to
the current is large and the energy of the battery is expended in heat
rather than in current. If the heat thus produced is very great,
serious consequences may arise; for example, the contact of a hot wire
with wall paper or dry beams may cause fire. Insurance companies
demand that the wires used in wiring a building for electric lights be
of a size suitable to the current to be carried, otherwise they will
not take the risk of insurance. The greater the current to be carried,
the coarser is the wire required for safety.

289. Electric Stoves. It is often desirable to utilize the electric
current for the production of heat. For example, trolley cars are
heated by coils of wire under the seats. The coils offer so much
resistance to the passage of a strong current through them that they
become heated and warm the cars.

[Illustration: FIG. 201.--An electric iron on a metal stand.]

Some modern houses are so built tha