Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . o about 5 amperes. Thechief advantages of inclosed arcsare the saving of carbon and thediminished cost of labor for trim-ming. An open-air arc is trimmednightly and an inclosed lamp, prob-ably once every ten days. 378. Alternating CurrentArcs.—In arcs fed from an alter-nating current there is not a con-tinuous flame, but the arc is lightedand extinguished with each reversalof current. When the reversals are greater than 100 persecond the flicker is not ordinarily noticeable. There is
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . o about 5 amperes. Thechief advantages of inclosed arcsare the saving of carbon and thediminished cost of labor for trim-ming. An open-air arc is trimmednightly and an inclosed lamp, prob-ably once every ten days. 378. Alternating CurrentArcs.—In arcs fed from an alter-nating current there is not a con-tinuous flame, but the arc is lightedand extinguished with each reversalof current. When the reversals are greater than 100 persecond the flicker is not ordinarily noticeable. There is nocrater, since each carbon acts as a positive at every otheralternation, and the carbon consumption is nearly the same,*(\ 375. Cored carbons are always used in alternating currentlamps to secure a sufficient supply of the carbon vapor tomaintain the currents path at the instant the arc is ex-tinguished, when the current reverses. For this reason thecurrent required for alternating current arcs must be largerthan for direct current arcs, usually about 15 amperes, with30 to 35 volts drop across the Fig. 382.—Single Carbon Automatic Feed Inclosed Arc Lamp. The arc is inclosed in the inner globe and the lamp will burn for 150 hours with one set of carbons. 422 PRACTICAL ELECTRICITY. 379. Arc Lamp Circuits.—Arc lamps are operated fromconstant potential or constant current circuits, and suppliedby either direct or alternating currents. Special features ofmechanical construction are embodied in the lamps accord-ing to the circuit from which they are to be supplied. Whenit is required to operate 45-volt series arc lamps from 110-voltconstant potential circuits, two lamps are connected in serieswith sufficient extra resistance to give the proper drop acrossthe lamps. Resistance is also added in series with inclosedlamps so that one lamp may be operated from a 110-voltconstant potential main, or two in series, with extra resistanceplaced across 220 volts. Choke coils, % 29
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