. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. yed,and the resistance (alwayssupplied with the laimp)enclosed in a glass bulbbehind. Each of these fila-ments carries roughly one ampere on a 200-volt current, and by usinga lamp with three of them as much as 1,000-candle power cam beobtained, but owing to the length of these filaments the effective light,as well as the definition, are by no means equal to the electric peculiarity of these lamps is that no current can be passedthrough them until the filaments are heated. This can be
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. yed,and the resistance (alwayssupplied with the laimp)enclosed in a glass bulbbehind. Each of these fila-ments carries roughly one ampere on a 200-volt current, and by usinga lamp with three of them as much as 1,000-candle power cam beobtained, but owing to the length of these filaments the effective light,as well as the definition, are by no means equal to the electric peculiarity of these lamps is that no current can be passedthrough them until the filaments are heated. This can be dome bymeans of an automatic arrangement, but for lantern work it isusually done by hand, holding a spirit lamp (specially supplied fortho purpose) under the lamp for a few moments. Once the current is started the spirit lamp may be removed, asthe filaments, of course, keep hot until the current is switched this is done for any reason, the spirit lamp must again be appliedbefore current can pass through the filaments again. Except, however, for the trouble entailed by this initial starting,. Fig. 239.—Nernst Lamp. APPENDIX 42$ the lamps are extremely simple and easy to manage, and thelight is absolutely steady and uniform. A filament will occasionally break, but if a few spare ones arekept in stock this is easily replaced, a pair of forceps and a littlepatience being all the tools needed. Electric Arc Lamps—The advance in public lighting has pro-duced many cheap and simple pattern arc lamps for lantern use. Those most commonly employed are of the hand-fed type, arack or screw adjustment moving the carbons towards or awayfrom each other. Fig. 240 shows a typical pattern.
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