charged by a current of
electricity, a chemical change is gradually produced in the active
element of which the accumulator is made. This change or decomposition
continues so long as the charging current is on. When the accumulator is
disconnected from the charging battery or dynamo, and its terminals are
connected up with a lighting system, or with a motor, for instance, a
reverse process is set up, or the particles re-form themselves into
their original compositions, which causes a current to flow in a
direction opposite to that of the charging current.

It is immaterial to the purposes of this chapter, as to the charging
source, whether it be by batteries or dynamos; the same principles will
apply in either case.

[Illustration: _Fig. 62._ ACCUMULATOR GRIDS]

ACCUMULATOR PLATES.--The elements used for accumulator plates are red
lead for the positive plates, and precipitated lead, or the well-known
litharge, for the negative plates. Experience has shown that the best
way to hold this material is by means of lead grids.

Fig. 62 shows the typical form of one of these grids. It is made of
lead, cast or molded in one piece, usually square, as at A, with a wing
or projection (B), at one margin, extending upwardly and provided with a
hole (C). The grid is about a quarter of an inch thick.

THE GRID.--The open space, called the grid, proper, comprises cross
bars, integral with the plate, made in a variety of shapes. Fig. 62
shows three forms of constructing these bars or ribs, the object being
to provide a form which will hold in the lead paste, which is pressed in
so as to make a solid-looking plate when completed.

THE POSITIVE PLATE.--The positive plate is made in the following manner:
Make a stiff paste of red lead and sulphuric acid; using a solution,
say, of one part of acid to two parts of water. The grid is laid on a
flat surface and the paste forced into the perforations with a stiff
knife or spatula. Turn over the grid so as to get the paste in evenly on
both sides.

The grid is then stood on its edge, from 18 to 20 hours, to dry, and
afterwards immersed in a concentrated solution of chloride of lime, so
as to convert it into lead peroxide. When the action is complete it is
thoroughly rinsed in cold water, and is ready to use.

THE NEGATIVE PLATE.--The negative plate is filled, in like manner, with
precipitated lead. This lead is made by putting a strip of zinc into a
standard solution of acetate of lead,