. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. 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 ad
. Optical projection : a treatise on the use of the lantern in exhibition and scientific demonstration. 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 Fig. 240.—Hand-fed Arc Lamp Two screws, a and b, serve for centring the light, c is a rackmovement for bringing the carbons gradually together as they burnaway, d is a sliding movement for adjusting the position of the topcarbon with regard to the lower. It will be seen that the plan ofslanting the carbons, alluded to in Chapter XII., has beenadhered to. These lamps are usually constructed to take carbons of varyingsizes, in accordance with the amount of current intended to be used,and the plan is usually followed of having the upper or positivecarbon, which burns away quickest, considerably the larger of thetwo, so that they shall waste at equal rates, and thereby keep thearc approximately in the optical centre of the lantern. For acurrent of 10 amperes, convenient sizes are about 13 mm. for the 426 OPTICAL PROJECTION positive carbon, which should be cored, have a soft centtre, and8 mm. for the negative, which should be a solid or entirely hard carbon. It must be rememberec?th
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