for farms, but is given merely to show how a farmer may
figure his own requirements. In some instances, it will be necessary
to install a battery of 120 or more ampere hours, whereas a battery of
40 or 60 ampere hours would be quite serviceable in other instances.
It all depends on how much light you wish to use and are willing to
pay for, because with a storage battery the cost of electric light is
directly in proportion to the number of lights used.

As a general rule, a larger generator and engine are required for a
larger battery--although it is possible to charge a large battery
with a small generator and engine by taking more time for the
operation.

_How to Charge a Storage Battery_

Direct current only can be used for charging storage batteries. In the
rare instance of alternating current only being available, it must be
converted into direct current by any one of the many mechanical,
chemical, or electrical devices on the market--that is, the
alternating current must be straightened out, to flow always in one
direction.

A shunt-wound dynamo must be used; else, when the voltage of the
battery rises too high, it may "back up" and turn the dynamo as a
motor, causing considerable damage. If a compound dynamo is already
installed, or if it is desired to use such a machine for charging
storage batteries, it can be done simply by disconnecting the series
windings on the field coils, thus turning the machine into a shunt
dynamo.

The voltage of the dynamo should be approximately 50 per cent above
the working pressure of the battery. For this reason 45-volt machines
are usually used for 30 or 32-volt batteries. Higher voltages may be
used, if convenient. Thus a 110-volt dynamo may be used to charge a
single 2-volt cell if necessary, although it is not advisable.

_Direction of Current_

Electricity flows from the positive to the negative terminal. A
charging current must be so connected that the negative wire of the
dynamo is always connected to the negative terminal of the battery,
and the positive wire to the positive terminal. As the polarity is
always marked on the battery, there is little danger of making a
mistake in this particular.

When the storage battery is charged, and one begins to use its
accumulation of energy, the current comes out in the opposite
direction from which it entered in charging. In this respect, a
storage battery is like a clock spring, which is wound up in one
direction, a