nd unwinds itself in the other. With all storage battery
outfits, an ammeter (or current measure) is supplied with zero at the
center. When the battery is being charged, the indicating needle
points in one direction in proportion to the strength of the current
flowing in; and when the battery is being discharged, the needle
points in the opposite direction, in proportion to the strength of the
current flowing out.

Sometimes one is at loss, in setting about to connect a battery and
generator, to know which is the positive and which the negative wire
of the generator. A very simple test is as follows:

Start the generator and bring it up to speed. Connect some form of
resistance in "series" with the mains. A lamp in an ordinary lamp
socket will do very well for this resistance. Dip the two ends of the
wire (one coming from the generator, the other through the lamp) into
a cup of water, in which a pinch of salt is dissolved. Bring them
almost together and hold them there. Almost instantly, one wire will
begin to turn bright, and give off bubbles. The wire which turns
bright and gives off bubbles is the _negative_ wire. The other is the
positive.

[Illustration: A rough-and-ready farm electric plant, supplying two
farms with light, heat and power; and a Ward Leonard-type
circuit-breaker for charging storage batteries]

_Care of Battery_

Since specific directions are furnished with all storage batteries, it
is not necessary to go into the details of their care here. Storage
battery plants are usually shipped with all connections made, or
plainly indicated. All that is necessary is to fill the batteries with
the acid solution, according to directions, and start the engine. If
the engine is fitted with a governor, and the switchboard is of the
automatic type, all the care necessary in charging is to start the
engine. In fact, many makes utilize the dynamo as a "self-starter" for
the engine, so that all that is necessary to start charging is to
throw a switch which starts the engine. When the battery is fully
charged, the engine is stopped automatically.

The "electrolyte" or solution in which the plates of the lead battery
are immersed, is sulphuric acid, diluted with water in the proportion
of one part of acid to five of water, by volume.

The specific gravity of ordinary commercial sulphuric acid is 1.835.
Since its strength is apt to vary, however, it is best to mix the
electrolyte with the aid of the hydromete