HOW LIGHT IS PRODUCED BY THE ARC LAMP.
=152. The Electric Arc.= When a strong current passes from one carbon
rod to another across an air-space, an _electric arc_ is produced.
When the ends of two carbon rods touch, a current can pass from one to
the other, but the imperfect contact causes resistance enough to heat
the ends red-hot. If the rods be separated slightly, the current will
continue to flow, as the intensely heated air and flying particles of
carbon reduce the resistance of the air-space.
Fig. 206 shows two carbon rods which are joined to the two terminals
of a dynamo. The upper, or positive, carbon gradually wears away and
becomes slightly hollow. The heated _crater_, as it is called, is the
hottest part. The negative carbon becomes pointed. The arc will pass in
a vacuum, and even under water.
[Illustration: Fig. 206.]
As the electric arc is extremely hot, metals are easily vaporized in
it; in fact, even the carbon rods themselves slowly melt and vaporize.
This extreme heat is used for many industrial purposes.
[Illustration: Fig. 207.]
[Illustration: Fig. 208.]
"The phenomenon of the electric arc was first noticed by Humphrey
Davy in 1800, and its explanation appears to be the following: Before
contact the difference of potential between the points is insufficient
to permit a spark to leap across even 1/10000 of an inch of air-space,
but when the carbons are made to touch, a current is established.
On separating the carbons, the momentary extra current due to
self-induction of the circuit, which possesses a high electromotive
force, can leap the short distance, and in doing so volatilizes a small
quantity of carbon between the points. Carbon vapor, being a partial
conductor, allows the current to continue to flow across the gap,
provided it be not too wide; but as the carbon vapor has a very high
resistance it becomes intensely heated by the passage of the current,
and the carbon points also grow hot. Since, however, solid matter is a
better radiator than gaseous matter, the carbon points emit far more
light than the arc itself, though they are not so hot. It is observed,
also, that particles of carbon are torn away from the + electrode,
which becomes hollowed out to a cup-shape, and some of these are
deposited on the - electrode."
[Illustration: Fig. 209.]
=153. Arc Lamps.= As the carbons gradually wear away, some device is
necessary to keep their ends the right distance apart. If they are too
near, the arc is very small; and if too far apart, the current can not
pass and the light goes out. The positive carbon gives the more intense
light and wears away about twice as fast as the - carbon, so it is
placed above the - carbon, to throw the light downwards.
[Illustration: Fig. 210.]
[Illustration: Fig. 211.]
Arc lamps contain some device by which the proper distance between
the carbons can be kept. Most of them grip the upper carbon and pull
it far enough above the lower one to establish the arc. As soon as
the distance between them gets too great again, the grip on the upper
carbon is loosened, allowing the carbon to drop until it comes in
contact with the lower one, thus starting the current again. These
motions are accomplished by electromagnets. Fig. 207 shows a form of
arc lamp with _single carbons_ that will burn from 7 to 9 hours.
[Illustration: Fig. 212.]
[Illustration: Fig. 213.]
[Illustration: Fig. 214.]
Fig. 208 shows the mechanism by which the carbons are regulated. Fig.
209 shows a form of _double carbon_, or _all-night_ lamp, one set of
carbons being first used, the other set being automatically switched in
at the proper time.
[Illustration: Fig. 215.]
Figs. 210, 211 show forms of _short arc lamps_, for use under low
ceilings, so common in basements, etc.
Fig. 212 shows a _hand-feed focussing_ type of _arc lamp_. In regular
street lamps, the upper carbon only is fed by mechanism, as it burns
away about twice as fast as the lower one, thus bringing the arc lower
and lower. When it is desired to keep the arc at the focus of a
reflector, both carbons must be fed.
Fig. 213 shows a _theatre arc lamp_, used to throw a strong beam of
light from the balcony to the stage.
Fig. 214 shows the arc lamp used as a search-light. The reflector
throws a powerful beam of light that can be seen for miles; in
fact, the light is used for signalling at night. Fig. 215 shows how
search-lights are used at night on war-vessels.