[Electric engineering.] . and commercial work. In all cases where arc lamps are used in connection withmirrors or lenses for projection work, it is essential that thearc be kept in the focus of the mirror or lens. The lampsmust, therefore, be arranged to move the carbons towardseach other as they are consumed, and they must do this insuch a way that the position of the arc will not change. Alamp that does this is known as a focusing- larap. Forordinary lighting, it is not essential that the arc be kept inone place, so that the lower carbon is nearly always fixedand the arc maintained by allowi
[Electric engineering.] . and commercial work. In all cases where arc lamps are used in connection withmirrors or lenses for projection work, it is essential that thearc be kept in the focus of the mirror or lens. The lampsmust, therefore, be arranged to move the carbons towardseach other as they are consumed, and they must do this insuch a way that the position of the arc will not change. Alamp that does this is known as a focusing- larap. Forordinary lighting, it is not essential that the arc be kept inone place, so that the lower carbon is nearly always fixedand the arc maintained by allowing the upper one to movedownwards as the carbon is consumed. Fig. 11 shows a rather peculiar arrangement that is usedfor stereopticon lamps. Here the carbons are arranged at right angles to eachother. The lamp mech-anism moves B out ina horizontal direction,and C upwards as theyfig. H. burn away, so that the arc is always maintained in the same position at a. Theposition of C keeps it from interfering with the lens /, and. 14 ELECTRIC LIGHTING. §18 allows the greater part of the crater in the end of B to beexposed. 16. Composition of Carbons.—Carbons used for ordi-nary open-arc lamps in America are composed principally ofpetroleum coke. This is made from the residue left fromthe distillation of petroleum. It is ground up and mixedwith a binding material, such as tar, or a similar substance,and is then molded into rods. Sometimes the rods aremade in molds under a heavy pressure, but more frequentlythey are made by forcing the material through dies. Therods are then gradually dried and afterwards baked or firedat a high temperature. Gas-retort carbon has also beenused for the manufacture of arc-light carbons, the exactcomposition used varying with different makers. 17. For enclosed-arc lamps a very much finer quality ofcarbon is required than for the open-arc lamp. If the car-a bons used in these lamps are at all impure, theimpurities become volatilized and are deposited onthe
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