ILLl' MI \ATION AND PHOTOMETRY 



425 



With an untreated carbon filament the effect of increasing 

 the voltage is to decrease the resistance, as carbon has a nega- 

 tive temperature coefficient. Because of the decreased resist- 

 ance of the filament, the current increases more rapidly than the 

 voltage. Hence the power taken by the filament increases even 

 more rapidly than the voltage squared. This makes the carbon 

 lamp more sensitive to voltage changes than it otherwise would 



On the other hand, the metallized carbon filament and the 

 tantalum and tungsten filaments all have positive temperature 

 coefficients. An increased voltage is accompanied by increased 



unce which tends to prevent the lamp taking more power. 



Mct.-illizi-d-y! 



Tuntf-U-n 



'.'arbor. 



g 



Candle- 

 power 



Watts 

 per 



Candle 



bo 



95 100 105 



Per Cent. Rated Voltage 



110 



115 



^_'. KfTrrt <>f voltam- Change on eandlepowiT and watta per r:ui<ll<. 



, these last three types of lamps are less sensitive to 

 voltage changes than is the carbon lamp. Figure 382 shows 

 the variation of candlepower and elliciency with per cent, of 

 normal voltage for carbon and for tungsten lamps. 



192. Arc Lamps. The arc lamp was the first successful 

 iC lighting unit. It.- principle is the heating of the tips of 

 carbons, or other electrodes, to incandescence by means of an 

 electric current. This is illustrated by Fig. 388. Two carl>on 

 rods are connected in -cries across the light ing mains^a resistance 

 I\ being in series with the rods. If the carbon tips are first. 

 touched together, the heat developed at the point ol 



produce.- a hot vapor which immediately becomes conducting. 



It' the carbon- now be drawn apari tpor will form and this 



