negative electrons or corpuscles of
electricity. This temperature-difference was later taken into account in
designing direct-current arc-lamps, for inasmuch as most of the light
from an ordinary arc is emitted by the end of the positive electrode,
this was placed above the negative electrode. In this manner most of the
light from the arc is directed downward where desired. In the few
instances in modern times where the ordinary direct-current arc has been
used for indirect lighting, in which case the arc is above an inverted
shade, the positive carbon is placed below the negative one. Gassiot
first proved that the positive electrode is hotter than the negative one
by striking an arc between the ends of two horizontal wires of the same
substance and diameter. After the arc operated for some time, the
positive wire was melted for such a distance that it bent downward, but
the negative remained quite straight.

Charcoal was used for the electrodes in all the early experiments, but
owing to the intense heat of the arc, it burned away rapidly. A
progressive step was made in 1843 when electrodes were first made by
Foucault from the carbon deposited in retorts in which coal was
distilled in the production of coal-gas. However, charcoal, owing to its
soft porous character, gives a longer arc and a larger flame. In 1877
the "cored" carbons were introduced. These consist of hard molded carbon
rods in which there is a core of soft carbon. In these are combined the
advantages of charcoal and hard carbon and the core in burning away more
rapidly has a tendency to hold the arc in the center. Modern carbons for
ordinary arc-lamps are generally made of a mixture of retort-carbon,
soot, and coal-tar. This paste is forced through dies and the carbons
are baked at a fairly high temperature. A variation in the hardness of
the carbons may be obtained as the requirements demand by varying the
proportions of soot and retort-carbon. Cored carbons are made by
inserting a small rod in the center of the die and the carbons are
formed with a hollow core. This may be filled with a softer carbon.

If two carbons connected to a source of electric current are brought
together, the circuit is completed and a current flows. If the two
carbons are now slightly separated, an arc will be formed. As the arc
burns the carbons waste away and in the case of direct current, the
positive decreases in length more rapidly than the negative one. This is
due large