Untitled

CHAPTER VIII.
he Electric iiigbt.
THE electric light has been applied to the lantern both in
the form of the incandescent and of the arc lamp. In the
former case, with a
view to keep down
the size of the lightemitting
surface, a
special type of incandescent
lamp has
been invented and
supplied by the Edison
Swan Co., which is
shown in Fig. 33.
In this it will be seen
that the filament, as
the little carbon
thread which emits
the light is termed,
is in the form of a
closely coiled helix.
Such lamps are made
usually either of 50 or
100 candle-power, and
Fig. 33. INCANDESCENT LAMP FOR THE LANTERN. the light being very
concentrated, as much
can be got out of a 50 candle-power lamp of this kind
as out of a very much more powerful petroleum lamp.
THE ELECTRIC LIGHT.
'Where a very intense light is desired, one of these lamps
can be "overrun," that is to say, can be put on a circuit of
greater pressure than that for which the lamp is intenited.
The light emitted increases in a very rapid ratio, and soon
becomes as bright as any limelight jet, but the life of the
lamp is much shortened, a new one being required much
sooner than would otherwise be the case. This overruning
is most conveniently done by the use of what is known
as a variable resistance, shown in Fig. 34. By moving the
handle A to the right, the coils are gradually taken out of the
Fig. 34. DIAGRAM OF THE CONNECTIONS FOR
INCANDESCENT LAMP AND VARIABLE RESISTANCE.
circuit, and in consequence more current passes through
the lamp, with the result that the light emitted increases,
becoming at the same time much whiter in colour. Neither
this nor the arc light can be run except at a prohibitive cost
from batteries, but where the current is laid on they are
convenient.
A neat form of holder for these lamps is a plain socket
in which slides a brass tube attached to the stem of the
lamp, the lamp being held at any elevation, as shown in
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