ELECTRIC LIGHTING. 765 ELECTRIC LIGHTING. coke, ami laiiiptilaek. luit ^■lU'ially fiom the first. The crude material is erusheii. hij;hly hoateil, ground tine, bolted, in succession, then mixed warm with pitch: the compound is cooled, then reduced to powder, as before. It then may be A B BRUSH ARC LAMP. A. Ready for use. B. Estendeil for trimming. molded or forced through holes to give the desired cylindrical form. When molded the carbon is heated, then pressed heavily before being taken from the mold. The molded carbons are used principally in constant direct-current series lamps on account of their cheapness. The forced car- bons are employed for constant potential or alter- nating current lamps, where a uniform shape and higher crade of goods generally is desired. They mav be cored by making a hole about Vk inch in diameter in their centre and filling it with a special mixture, of less density than the body, to increase the steadiness of the arc. Imported carbons are generally molded and of a better grade than those made in America. Copper-jilaled carbons are nsed for high pres- sure, constant-current lamps in series, on account of their increased conductivity and long life. Ol.oiits for are lamps are made of clear, opal, or alabaster glass, and are more regular in thick- ness and shape when molded than when blown. The best thin clear silass globes transmit nO to 95 per cent, of light ; opal, from .50 to SO per cent. : alabaster. 40 to (50 per cent. The opal globes diffuse the light and cut off objectionable colors; clear globes are sometimes ground for the same reason. Holophane globes have special- ly cut outer surfaces, forming a variety of planes and prisms to eti'ect the most perfect diffusion possible, throwing the rays downward. Regi l.vtim: Dkvices are rciiuircil to feed the carbons so tluit the are is maintained constant as the carbon burns away. Open-are direct-cur- rent lamps are generally made with the lower carbon stationary and the upper one to feed dowiiward. The feeding is operated either by a train of wheelwork, electrical or mechanical motors, gravity, or the action of solenoids. Wbcn the lamp is not in operation the upper carbon falls and lests upon the lower one; but when the current is passed tlirough them the carbons are separated (he required distance by means of an electro-magnet whoso coils arc traversed by the whole current of the lamp. In order to maintain the carbons the ])roper distance apart, the upper carbon is held by a clutch or other device whose position is controlled by an electro-magnet. The coils of this electro-magnet are a shunt or branch of the main circuit, of high resistance. When the carbons are at their normal distance apart, the current of the shunt circuit is not of suflScient strength to move the clutcli from the position in A B INCLOSED ABC LIGHT. A. Direct current. B. .-ilteruatintf current. which it prevents the downward motion of the carbon rod ; but when the carbon has sulficiently liurned away to increase the resistance of the arc 1o a determined extent, the increased current which is tbereliy produced through (lie shunt cir- cuit is then suriiciently strong to release the clutch and permit the carbon to feed downward. In a well-regulated lamp this feeding occurs so
