Untitled

46 MODERN MAGIC LANTERNS.
Fig. 35, by the screw s. The holder shown in Fig. 35
should be secured by screws to a board cut to slide in
the grooves in the lantern, so as to be readily centered. In
Fig. 34 (a diagram of the connections for the incandescent
lamp), the variable resistance, B, can
be dispensed with if the lamp is only
to be run at its normal brilliancy, in
favour of a fixed resistance, or if the
current is supplied at the pressure required
by the lamp the resistance can
be done away with altogether. The
connections with the mains are shown
at c c.
The arc lamp is, however, the form
of electric light which seems peculiarly
adapted for projection purposes,
Fig. 35. HOLDER FOR
INCANDESCENT LAMP.
since it fulfils two important conditions,
a most intense light with the smallest
possible area. The arc lamp is so
called because the light is caused by the
current passing across an air space or arc between two carbon
points, which are, by it, heated to a great brilliancy. The arc
has first to be "struck," that is, the carbons must touch each
other when the current is first switched on, and have then to
be separated to the required distance to form the arc. As
the light is emitted the carbons gradually burn away, the
positive at about twice the rate of the negative ; means
have, in consequence, to be provided to keep their ends, or
poles, at an uniform distance apart, so that the arc may be
maintained.
Arc lamps may be divided into two classes, according to the
way this uniform distance is maintainedautomatic when
the current itself actuates or regulates the "feed " of the
carbons, and "hand fed" when this is done by the
operator. Both kinds are made in a convenient form for the
lantern, but as hand-fed lamps do not require much attention
from the lanternist, who is, moreover, on the spot to
give them that little, and as they are much cheaper and less
likely to get out of order, they are to be preferred. Fig.
36 is a simple type of automatic lamp, the "Scissors," by
THE ELECTRIC LIGHT.
Borland, of Leeds, it is also made for hand feed ; several
other patterns are on the market which space precludes us
from describing. Of hand-fed lamps, Davenport's, made by
Messrs. Steward and Co., though in the writer's hands for
but a short time, has so far proved satisfactory. It is shown
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'LA "II 141.0Ls. LEEDS.
Fig. 36. THE "SCISSORS" AUTOMATIC ARC LAMP.
in Fig. 7. The upper is the positive carbon, which is
gradually fed down by the rod and handle seen projecting
at the back, the lower or negative carbon being pushed up by
a spring against two screws, which prevent it going too far.
To get the best light the end of the negative carbon should
be a little nearer the condenser than the other, and the
arc should not be too long. If too long, a flame will be
seen to play round it ; if too short, it will probably "sing."
When the carbons have burned for a little time it will be
seen that while the negative has come to a rounded point
in shape, the positive has a little bowl or crater at its end,
and it is from the inside of this crater that the most intense
light is emitted: It is for this reason that the arc lamp for
47