426 



OPTICAL PROJECTION 



positive carbon, which should be cored, i.e. have a soft centre, and 

 8 mm. for the negative, which should be a solid or entirely hard 



carbon. It must be rememberec 1 

 that the electric arc only requires 

 about 45 volts, and as the street 

 mains usually give current at any- 

 thing from 100 to 250 volts' pressure, 

 some form of resistance must be 

 inserted. 



This is usually made of spirals of 

 iron, gerinan silver platinoid, or 

 other metal of high electrical resis- 

 tance, and may be a fixed amount, 

 suitable for the current to be need, 

 or adjustable. The latter is con- 

 venient, as it enables the operator 

 to vary the light if required. A 

 fixed resistance is, however, con- 

 siderably the cheaper, and for a 

 great deal of work answers perfectly 

 well. 



Fig. 241 shows an adjustable resistance constructed especially for 

 lantern work. The figure almost explains itself ; the frame is made 

 either of iron, or if required for portability, of aluminium. 



The following table gives the resistance required for various 

 currents and voltages and also suitable size carbons to employ for 

 arc lamps in small lanterns. 



APPROXIMATE TABLE OF BESISTANCE FOR VARIOUS CURRENTS AND SIZE 

 OF CARBONS. 



FIG. 241. Adjustable Resistance 



