CARRONS arc made from petroleum coke, gal; coke. and lampblack. but generally from the first. The crude material is crushed. highly heated, ground tine. bolted, in succession, then mixed warm with pitch: the compound is cooled. then reduced to powder, as before. It then may be colors; clear globes are sometimes ground for the same reason. Ilolophane globes have• special ly cut outer surfaces, forming a variety of planes and prisms to effect the most perfect diffusion possible. throwing the rays downward.
liEut'LATING Ds:vicEs are required to feed the carbons so that the arc is maintained constant as the carbon burns away. Open-arc direct-cur rent lamps are generally mole with the lower carbon stationary and the upper one to feed downward. The feeling is operated either by a train of •heelwork, electrical or mechanical motors. gravity. or action of solenoids. \\lien the lamp is not in operation the upper carbon falls and rests upon the lower one: but when the current is passed through them the carbons are separated the required distance by means of an electro-inaenet whose coils are traversed by the whole current of the lamp. In order to maintain the carbons the proper distance apart, the carbon is held by a clutch or other device whose position is controlled by an electromagnet. 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 sufficient strength to move the clutch from the position in 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-eurrent series lamps on account of their cheapness. The forced car bons arc employed for constant potential or alter nating current lamps, where a uniform shape and higher grade of goods generally is desired. They may be cored by making a hole about inch in diameter in their centre and tilling 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.
ropper-plated carbons are used for high pres sure. constant-current lamps in series, on account of their increased eondu•tivity and long life.
num., for are lamps are made of clear, opal. or alabaster 7lass, and are more regular in thick ness and shape when molded than when blown. The best thin clear class globes transmit 90 to 95 per rent. of light : opal. from 50 to RO per cent.: alabaster, 40 to 00 per cent. The opal globes diffuse the light and cut °IT objectionable which it prevents the downward motion of the carbon rod: lint when the carbon has sufficiently burned away to increase the resistance Of the arc to a determined extent, the increased current which is thereby produced through the shunt eir cnit is then sufficiently strong to release the clutch and permit the carbon to feed downward. In a well-regulated lamp this feeding occurs so gradually that it produces no perceptible effect on the steadiness. of the light. Double carbon lamps were introduced to increase the length of time the lamps would IMAM without renewing the car bons. In these lamps one pair of carbons is consumed before the other pair comes into use. Incloacd arc lamps are a recent improvement on the ordinary are lamp. which requires trimming every night. The carbons in the inclosed arc light will burn 100 to 120 hours or inure. The arc is inclosed in a globe or chamber made as nearly air-tight as possible, consistent with allowing the carbon to feed from the top of it. A small valve opening outward is connected with the chamber. When the lamp is lighted the intense heal of the arc expands the air in the chamber and forces out a part of it. through the valve, which also prevents the admission of outside air to the chamber. The carbons then burn in a rarefied atmosphere, and the leakage of air into the chamber is so small that the carbons are oxidized very slowly. The feeding mechanisms for inclosed arc lamps are simpler than for open arcs. Inclosed arcs may be used with constant current in series or with constant potential in parallels, and also with alternating currents.