APPENDIX 425 



the lamps are extremely simple and easy to manage, and the 

 light is absolutely steady and uniform. 



A filament will occasionally break, but if a few spare ones are 

 kept in stock this is easily replaced, a pair of forceps and a little 

 patience 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, a 

 rack or screw adjustment moving the carbons towards or away 

 from each other. Fig. 240 shows a typical pattern. 



FIG. 240. Hand-fed Arc Lamp 



Two screws, A and B, serve for centring the light, c is a rack 

 movement for bringing the carbons gradually together as they burn 

 away, D is a sliding movement for adjusting the position of the top 

 carbon with regard to the lower. It will be seen that the plan of 

 slanting the carbons, alluded to in Chapter XII., has been 

 adhered to. 



These lamps are usually constructed to take carbons of varying 

 sizes, in accordance with the amount of current intended to be used, 

 and the plan is usually followed of having the upper or positive 

 carbon, which burns away quickest, considerably the larger of the 

 two, so that they shall waste at equal rates, and thereby keep the 

 arc approximately in the optical centre of the lantern. For a 

 current of 10 amperes, convenient sizes are about 13 mm. for the 



