always kept fully charged gives it a long
life.

The same result can be achieved in storage battery plants for house
lighting, where the source of power is a gasoline or other engine
engaged normally in other work. Then your electric current becomes
merely a by-product of some other operation.

Take a typical instance where such a plant would be feasible: Farmer
Brown has a five horsepower gasoline engine--an ordinary farm engine
for which he paid probably $75 or $100. Electric light furnished
direct from such an engine would be intolerable because of its
constant flickering. This five horsepower engine is installed in the
milk room of the dairy, and is belted to a countershaft. This
countershaft is belted to the vacuum pump for the milking machine, and
to the separator, and to a water pump, any one of which may be thrown
into service by means of a tight-and-loose pulley. This countershaft
is also belted to a small dynamo, which runs whenever the engine is
running. The milking machine, the separator, and the water pump
require that the gasoline engine be run on the average three hours
each day.

The dynamo is connected by wires to the house storage battery through
a properly designed switchboard. The "brains" of this switchboard is
a little automatic device (called a regulator or a circuit breaker),
which opens and shuts according to the amount of current stored in the
battery and the strength of the current from the generator. When the
battery is "full," this regulator is "open" and permits no current to
flow. Then the dynamo is running idle, and the amount of power it
absorbs from the gasoline engine is negligible. When the "level" of
electricity in the battery falls, due to drawing current for light,
the regulator is "shut," that is, the dynamo and battery are
connected, and current flows into the battery.

These automatic instruments go still farther in their brainy work.
They do not permit the dynamo to charge the battery when the voltage
falls below a fixed point, due to the engine slowing down; neither do
they permit the dynamo current to flow when the voltage gets too high
due to sudden speeding up of the engine.

Necessarily, an instrument which will take care of a battery in this
way, is intricate in construction. That is not an argument against it
however. A watch is intricate, but so long as we continue to wind it
at stated intervals, it keeps time. So with this storage battery
plant: so long as Farm