gin to dim, when it
is switched in by means of a simple arrangement of connections. This
maintains a uniform pressure of 30 volts from the beginning to the end
of the charge, at the lamp socket.

We saw in earlier chapters that the 110-volt current is the most
satisfactory, under all conditions, where the current is to be used
for heating and small power, as well as light. But a storage battery
of 110 volts would require at least 55 cells, which would make it too
expensive for ordinary farm use. As a 30-volt current is just as
satisfactory for electric light, this type has become established, in
connection with the battery, and it is used for electric lighting
only, as a general rule.

Batteries are rated first, as to voltage; second, as to their capacity
in ampere hours--that is, the number of amperes that may be drawn from
them in a given number of hours. Thus, a battery rated at 60 ampere
hours would give 60 amperes, at 30 volts pressure, for one hour; 30
amperes for 2 hours; 15 amperes for 4 hours; 7-1/2 amperes for 8
hours; 3-3/4 amperes for 16 hours; etc., etc. In practice, a battery
should not be discharged faster than its 8-hour rate. Thus, a
60-ampere hour battery should not be drawn on at a greater rate than
7-1/2 amperes per hour.

This 8-hour rate also determines the rate at which a battery should
be re-charged, once it is exhausted. Thus, this battery should be
charged at the rate of 7-1/2 amperes for 8 hours, with another hour
added to make up for losses that are bound to occur. A battery of
120-ampere hour capacity should be charged for 8 or 9 hours at the
rate of 120 / 8, or 15 amperes, etc.

To determine the size of battery necessary for any particular
instance, it is necessary first to decide on the number of lamps
required, and their capacity. Thirty-volt lamps are to be had in the
market in sizes of 10, 15 and 20 watts; they yield respectively 8, 12,
and 16 candlepower each. Of these the 20-watt lamp is the most
satisfactory for the living rooms; lamps of 10 or 15 watts may be used
for the halls, the bathroom and the bedrooms. At 30 volts pressure
these lamps would require a current of the following density in
amperes:

Candle
Power 30-volt lamp Amperes
8 10 watts 0.33
12 15 watts 0.50
16 20 watts 0.67

Let us assume, as an example, that Farmer Brown will use 20-watt
lamps in his kitchen, dining room, and sitt