electrolyte, the
gravity readings as obtained with the hydrometer syringe should be
corrected one point for every three degrees change in temperature.
Thus 70 deg.F is considered the normal temperature, and one point is added
to the electrolyte reading for every three degrees above 70 deg.F.
Similarly, one point is subtracted for every three degrees below 70 deg.F.
For convenience of the hydrometer user, a special thermometer has been
developed by battery makers. This is shown in Fig. 37. It has a
special scale mounted beside the regular scale. This scale shows the
corrections which must be made when the temperature is not 70 deg.F.
Opposite the 70 deg. point on the thermometer is a "0" point on the
special scale. This indicates that no correction is to be made.
Opposite the 67 deg. point on the regular scale is a -1, indicating that 1
must be subtracted from the hydrometer reading to find what the
specific gravity would be if the temperature were 70 deg.F. Opposite the
73 deg. point on the regular scale is a +1, indicating that 1 point must
be added to reading on the hydrometer, in order to reduce the reading
of specific gravity to a temperature of 70 deg.F.

[Fig. 37 Special thermometer]

8. Storage batteries are strongly affected by changes in temperature.
Both extremely high and very low temperatures are to be avoided. At
low temperatures the electrolyte grows denser, the porosity of plates
and separators decreases, circulation and diffusion of electrolyte are
made difficult, chemical actions between plates and acid take place
very slowly, and the whole battery becomes sluggish, and acts as if it
were numbed with cold. The voltage and capacity of the battery are
lowered.

As the battery temperature increases, the density of the electrolyte
decreases, the plates and separators become more porous, the internal
resistance decreases, circulation and diffusion of electrolyte take
place much more quickly, the chemical actions between plates and
electrolyte proceed more rapidly, and the battery voltage and capacity
increase. A battery therefore works better at high temperatures.

Excessive temperatures, say over 110 deg. F, are, however, more harmful
than low temperatures. Evaporation of the water takes place very
rapidly, the separators are attacked by the hot acid and are ruined,
the active materials and plates expand to such an extent that the
active materials break away from the grids and the grids warp and
buc