battery hydrometers which are marked "Full," "Empty,"
"Charged," "Discharged," must not be used, since the specific
gravities corresponding to these words are not the same in farm
lighting batteries as in automobile batteries and the readings would
be incorrect and misleading. If the manufacturer-of the battery
furnishes a special hydrometer which is marked "Full," "Half-Full,"
"Empty," or in some similar manner, this hydrometer may, of course, be
used.

Temperature corrections should be made in taking hydrometer readings,
as described on page 65. For Prest-O-Lite batteries, 80 degrees is the
standard temperature, and gravity readings on these batteries should
be corrected to 80 degrees as described on page 461.

Gravity readings should, of course, be taken during charge as well as
during discharge. The readings taken during charge are described in
the following sections on charging.

Charging

(See also special instructions for the different makes, beginning page
460.)

Two kinds of charges should be given the battery, the "Regular"
charge, and the "Overcharge" or "Equalizing Charge." These will be
spoken of as the "Regular" charge and the "Overcharge." The Regular
charge must be given whenever it is necessary in order to enable the
battery to meet the lighting or other load demands made upon it. The
overcharge, which is merely a continuation of a regular charge, should
be given once every month. The overcharge is given to keep the battery
in good condition, and to prevent the development of inequalities in
condition of cells.

When to Charge. Experience will soon show how often you must give a
regular charge in order to keep the lights from becoming dim. When the
voltage reading, taken while all the lamps are on has dropped to 1.8
volts per cell a Regular charge is necessary. When the specific
gravity of the pilot cell indicates that the battery is discharged, a
Regular charge is necessary. It is better to use the specific gravity
readings as a guide, as described later.

A good plan, and the best one, is to give a battery a Regular charge
once every week, whether the battery becomes discharged in one week's
time or not. A regular charge may be required oftener than once a
week. Every fourth week give the Overcharge instead of the Regular
charge.

If a battery is to be out of service, arrangements should be made to
add the necessary water and give an overcharge every month, the
Regular charges not bein