ells, the chemical method is used in preference to
the mechanical one. The numerous types of cells in daily use differ
chiefly in the devices employed for preventing the formation of
hydrogen bubbles, or for disposing of them when formed. One of the
best-known cells in which weakening of the current is prevented by
chemical means is the so-called gravity cell.

285. The Gravity Cell. A large, irregular copper electrode is placed
in the bottom of a jar (Fig. 198), and completely covered with a
saturated solution of copper sulphate. Then a large, irregular zinc
electrode is suspended from the top of the jar, and is completely
covered with dilute sulphuric acid which does not mix with the copper
sulphate, but floats on the top of it like oil on water. The hydrogen
formed by the chemical action of the dilute sulphuric acid on the zinc
moves toward the copper electrode, as in the simple voltaic cell. It
does not reach the electrode, however, because, when it comes in
contact with the copper sulphate, it changes places with the copper
there, setting it free, but itself entering into the solution. The
copper freed from the copper sulphate solution travels to the copper
electrode, and is deposited on it in a clean, bright layer. Instead of
a deposit of hydrogen there is a deposit of copper, and falling off in
current is prevented.

The gravity cell is cheap, easy to construct, and of constant
strength, and is in almost universal use in telegraphic work.
Practically all small railroad stations and local telegraph offices
use these cells.

[Illustration: FIG. 199.--A dry cell.]

286. Dry Cells. The gravity cell, while cheap and effective, is
inconvenient for general use, owing to the fact that it cannot be
easily transported, and the _dry cell_ has largely supplanted all
others, because of the ease with which it can be taken from place to
place. This cell consists of a zinc cup, within which is a carbon rod;
the space between the cup and rod is packed with a moist paste
containing certain chemicals. The moist paste takes the place of the
liquids used in other cells.

[Illustration: FIG. 200.--A battery of three cells.]

287. A Battery of Cells. The electromotive force of one cell may not
give a current strong enough to ring a door bell or to operate a
telephone. But by using a number of cells, called a battery, the
current may be increased to almost any desired strength. If three
cells are arranged as in Figure 200, so th