in some cases so
weak as to be worthless for practical use, such as telephoning, or
ringing a door bell. What is wanted is a strong, steady current, and
our choice of material is limited to the substances which will give
this result. Zinc and lead can be used, but the current resulting is
weak and feeble, and for general use zinc and carbon are the most
satisfactory.

283. Electrical Terms. The plates or strips used in making an
electric cell are called electrodes; the zinc is called the negative
electrode (-), and the carbon the positive electrode (+); the current
is considered to flow through the wire from the + to the-electrode. As
a rule, each electrode has attached to it a binding post to which
wires can be quickly fastened.

The power that causes the current is called the electromotive force,
and the value of the electromotive force, generally written E.M.F., of
a cell depends upon the materials used.

When the cell consists of copper, zinc, and dilute sulphuric acid, the
electromotive force has a definite value which is always the same no
matter what the size or shape of the cell. But the E.M.F. has a
decidedly different value in a cell composed of iron, copper, and
chromic acid. Each combination of material has its own specific
electromotive force.

284. The Disadvantage of a Simple Cell. When the poles of a simple
voltaic cell are connected by a wire, the current thus produced
slowly diminishes in strength and, after a short time, becomes feeble.
Examination of the cell shows that the copper plate is covered with
hydrogen bubbles. If, however, these bubbles are completely brushed
away by means of a rod or stick, the current strength increases, but
as the bubbles again gather on the + electrode the current strength
diminishes, and when the bubbles form a thick film on the copper
plate, the current is too weak to be of any practical value. The film
of bubbles weakens the current because it practically substitutes a
hydrogen plate for a copper plate, and we saw in Section 282 that a
change in any one of the materials of which a cell is composed changes
the current.

This weakening of the current can be reduced mechanically by brushing
away the bubbles as soon as they are formed; or chemically, by
surrounding the copper plate with a substance which will combine with
the free hydrogen and prevent it from passing onward to the copper
plate.

[Illustration: FIG 198. The gravity cell.]

In practically all c