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 largely to the extremely high temperature of the positive tip, where the carbon fairly boils.  A crater is formed at the positive tip and this is always characteristic of the positive carbon of the ordinary arc, although it becomes more shallow as the arc-length is increased.  The negative tip has a bright spot to which one end of the arc is attached.  By wasting away, the length of the arc increases and likewise its resistance, until finally insufficient current will pass to maintain the arc.  It then goes out and to start it the carbons must be brought together and separated.  The mechanisms of modern arc-lamps perform these functions automatically by the ingenious use of electromagnets.