tor, while the other consists of a thin sheet of hard rubber
containing many fine perforations. This rubber sheet is placed between
the positive plate and the wooden separator. A recent development in
the use of an auxiliary rubber separator is the Philco slotted
retainer which is placed between the separators and the positives in
Philadelphia Diamond Grid Batteries. Some Exide batteries also use
slotted rubber separators. The Philco slotted retainer consists of a
thin sheet of slotted hard rubber as shown in Fig. 9. The purpose of
the retainer is to hold the positive active material in place and
prevent the shedding which usually occurs. The slots in the retainer
are so numerous that they allow the free passage of electrolyte, but
each slot is made very narrow so as to hold the active material in the
plates.

Electrolyte

Little need be said here about the electrolyte, since a full
description is given elsewhere. See page 222. Acid is received by the
battery manufacturer in concentrated form. Its specific gravity is
then 1.835. The acid commonly used is made by the "contact" process,
in which sulphur dioxide is oxidized to sulphur trioxide, and then,
with the addition of water, changed to sulphuric acid. The
concentrated acid is diluted with distilled water to the proper
specific gravity.

Jars

The jars which contain the plates, separators, and electrolyte are
made of a tough, hard rubber compound. They are made either by the
moulding process, or by wrapping sheets of rubber compound around
metal mandrels. In either case the jar is subsequently vulcanized by
careful heating at the correct temperature.

The battery manufacturers do not, as a rule, make their own jars, but
have them made by the rubber companies who give the jars a high
voltage test to detect any flaws, holes, or cracks which would
subsequently cause a leak. The jars as received at the battery maker's
factory are ready for use.

Across the bottom of the jar are several stiff ribs which extend up
into the jar so as to provide a substantial support for the plates,
and at the same time form several pockets below the plates in which
the sediment resulting from shedding of active material from the
plates accumulates.

Covers

No part of a battery is of greater importance than the hard rubber
cell covers, from the viewpoint of the repairman as well as the
manufacturer. The repairman is concerned chiefly with the methods of
sealing the