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 invented an arc-lamp and an efficient form of dynamo to supply the electrical energy.  The first arc-lamps were ordinary direct-current open arcs and the carbons were made from high-grade coke, lampblack, and syrup.  The upper positive carbon in these lamps is consumed at a rate of one to two inches per hour.  Inasmuch as about 85 per cent. of the total light is emitted by the upper (positive) carbon and most of this from the crater, the lower carbon is made as small as possible in order not to obstruct any more light than necessary.  The positive carbon of the open arc is often cored and the negative is a smaller one of solid carbon.  This combination operates quite satisfactorily, but sometimes solid carbons are used outdoors.  The voltage across the arc is about 50 volts.

In 1846 Staite discovered that the carbons of an arc enclosed in a glass vessel into which the air was not freely admitted were consumed less rapidly than when the arc operated in the open air.  After the appearance of the dynamo, when increased attention was given to the development of arc-lamps, this principle of enclosing the arcs was again considered.  The early attempts in about 1880 were unsuccessful because