Article October 2, 1902 Issue

The Enclosed Electric Arc

Science and Industry

In the older style of lamp the arc is formed between carbon points to which the air has free access. The upper carbon is the positive, so that the current flows from the upper to the lower. The carbons become pointed after the are has burned a few minutes and the upper carbon burns about twice as fast as the lower, a small cup-shaped spot or crater, forming in it. The bottom carbon is much more pointed than the upper and does not attain to such a high temperature. The greater part of the light is emitted from the crater, and is thrown downward at an angle of about forty-five degrees. The temperature of the arc is exceedingly high and difficult to measure accurately, but it is estimated to be about 3,500 degrees C. . . . The chief objection to the open arc is that the carbons burn away at a comparatively rapid rate, thus making it necessary to trim the lamps daily. The positive carbon, if of the ordinary one-half-inch or nine-sixteenth-inch ·size, will burn about one and one half inches per hour, though, of course, the rate of burning depends somewhat on the hardness of the carbon. Also, the light from the open arc is not suitable for interior illumination, as it has too much of a glare unless it is toned down by means of opal globes or other devices that generally cut off at least fifty or sixty per cent of the light.

A great step in advance was made by the introduction of the enclosed arc lamp, which ... is now widely used both for indoor and outdoor illumination. The chief point of difference between the two lamps is that the newer style has its arc enclosed in a small globe which, while not air-tight, keeps the air away from the carbon to such an extent that the rate of carbon consumption is greatly decreased.

The result of enclosing the arc as above described is that most of the oxygen present is soon burned out, and the arc burns in a hot atmosphere of carbon monoxid, nitrogen, and a small amount of oxygen. It is necessary to have a small amount of oxygen present in order to combine with the small amount of carbon given off, and thus prevent its being deposited on the enclosing globe.

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