ical would be the light. Edison
sought, thirty years ago, for just the qualities now found in tungsten
metal. Tungsten metal was first used for incandescent lamps in the
form of a paste, squirted into the shape of a thread. This proved too
fragile. Later investigators devised means of drawing tungsten into
wire; and it is tungsten wire that is now used so generally in
lighting. A tungsten lamp has an average efficiency of 1-1/4 watts per
candlepower, compared with 3-1/2 to 4 watts of the old-style carbon
lamp. In larger sizes the efficiency is as low as .9 watt per
candlepower; and only recently it has been found that if inert
nitrogen gas is used in the glass bulb, instead of using a high vacuum
as is the general practice, the efficiency of the lamp becomes still
higher, approaching .5 watt for each candlepower in large lamps. This
new nitrogen lamp is not yet being manufactured in small domestic
sizes, though it will undoubtedly be put on the market in those sizes
in the near future.

[Illustration: The Fairbanks Morse oil engine storage battery set]

The tungsten lamp, requiring only one-third as much electric current
as the carbon lamp, for the same amount of light, reduces the size
(and the cost) of the storage battery in the same degree, thus
bringing the storage battery within the means of the farmer. Some idea
of the power that may be put into a small storage battery is to be had
from the fact that a storage battery of only 6 volts pressure, such as
is used in self-starters on automobiles, will turn a motor and crank a
heavy six-cylinder engine; or it will run the automobile, without
gasoline, for a mile or more with its own accumulated store of
electric current.

_The Low Voltage Battery_

The 30-volt storage battery has become standard for small lighting
plants, since the introduction of the tungsten lamp. Although the
voltage of each separate cell of this battery registers 2.5 volts when
fully charged, it falls to approximately 2 volts per cell immediately
discharging begins. For this reason, it is customary to figure the
working pressure of each cell at 2 volts. This means that a 30-volt
battery should consist of at least 15 cells. Since, however, the
voltage falls below 2 for each cell, as discharging proceeds, it is
usual to include one additional cell for regulating purposes. Thus,
the ordinary 30-volt storage battery consists of 16 cells, the last
cell in the line remaining idle until the lamps be