ELECTRIC LIGHTING. 766 graduaU' that it i)roiIuces no ppiceptiblc effect on the steadiness of the light. Douhle cart)on lamps ■were introduced to increase the length of time the lamps would burn without renewing the car- bons, in these lamps one ])air of earhons is consumed before the other pair comes into use. Jiiclosid arc laiiipn are a recent improvement on the ordinary arc lamp, which reiiuires trimming every night. The carbons in the inclosed arc light •will burn 100 to 120 hours or more. The arc is inclosed in a glol>e or chamber made as nearly air-tight as possible, consistent with allowing the carbon to feed from the top of it. A small valvo opening outward is connected with the chamber. When the lamp is lighted the intense heat of the arc ex])ands the air in the chamber and forces out a ]>art of it through the valve, which also jirevents 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 ctirrent in series or with constant potential in parallels, and also with alternating currents. lxc.>'DE.scEXT L.MPs arc very simple com- pared with arc lamps. The carbon strip or fila- ment is bent into the shape of a horseshoe, loop, or some more complex form, and placed inside of a glass bulb called the lamp-chamber. The air is then exhausted from the chamber, most commonly by pumping, after which the chamber is hermetically sealed. The exhaustion of the bulb should be as perfect as possible (see AiR- ELECTRIC LIGHTING. tion. Both the exhaustion and the heating of the lamp must be continued up to the moment that it is sealed. The ends of the filament are attached to platinum wires passing through one end of the glass bulb, and are called Icading-in wires. Filaments are prepared from numerous substances, such as silk, hair, wood -fibre, cotton. KORM8 OF F1LA.IENTS l.V I.SCANDKSCE.VT LAMPS, 1. SiiiKl*^ U. -■ Siiiigle cur] anchored. 3. Single curl, 4. Double curl anchored. 6. Double curl. 6. Double U. Pump), and in order to insure the complete re- moval of all the air in the bulb, and the occluded wascs in the carbon filament, both the bulb and the filament are heated to a high temperature during the latter part of the process of exhaus- AN INCANDESCENT LAMP IN PHOCESS OF MAEINS. 1. Thick tube ueed for base. 2. Second stapt' of base. 3. Base with platiiiuni wires, G, G. iuserted. The Ulanient is Joiued to the conducting wires at E and K nnil anchored at I'. 4. Bulb. .. blown from a Klass tube, 5, A tube, C, is attached to the upper end of the bulb. (>. The bull) with the base fused in. H is connected with an air-pump and the hulbcxhaustcd. 7. The finished lamp. after the bulb lias been sealed, aud the base inserted in its ni<»untinff fl'Mison form). (.>np terminal connects at I' with the metallic threaded en<l which tits into the socI<et. thus con- nectinj? with oue of tin' main wires. The other cnnnects with a projecting stud at U. which is imbedded in plaster of Paris, and thus insulated from the threaded iiart. This stud comes in contact: with an insulated strip iu the mechanism of the socl;et iu connt'ctiuu with one of the wires of the mains. cellulose. The most common practice now is to force a plastic solution of (cellulose through a hole, thus forming a thread. The filaments re- quire baking at high Icmpcrattircs. in (jrder to reduce them to pure carbon, their other constitu- ents being driven oil' by the heat. In some proc- esses of making filament s no treatment is re- (|iiired after the baking, as the filament is then liirned out perfectly even in cross-section through- out its length. Host filaments are not perfectly even in diameter over their whole length, and
