ly 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.

The interior of the arc is of a violet color and the exterior is a
greenish yellow. The white-hot spot on the negative tip is generally
surrounded by a fringe of agitated globules which consist of tar and
other ingredients of carbons. Often material is deposited from the
positive crater upon the negative tip and these accretions may build up
a rounded tip. This deposit sometimes interferes with the proper
formation of the arc and also with the light from the arc. It is often
responsible for the hissing noise, although this hissing occurs with any
length of arc when the current is sufficiently increased. The hissing
seems to be due to the crater enlarging under excessive current until it
passes the confines of the cross-section of the carbon. It thus tends to
run up the side, where it comes in contact with oxygen of the air. In
this manner the carbon is directly burned instead of being vaporized, as
it is when the hot crater is small and is protected from the air by the
arc itself. The temperature of the positive crater is in the
neighborhood of 6000 deg. to 7000 deg.F. The brightness of the arc under
pressure is the greatest produced by artificial means and is very
intense. By putting the arc under high pressure, the brightness of the
sun may be attained. The temperature of the hottest spot on the negative
tip is about a thousand degrees below that of the positive.

No great demand arose for arc-lamps until the development of the Gramme
dynamo in 1870, which provided a practicable source of electric current.
In 1876 Jablochkov invented his famous "electric candle" consisting of
two rods of carbon placed side by side but separated by insulating
material. In this country Brush was the pioneer in the development of
open arc-lamps. In 1877 he inve