PHOTOMETRY AND ELECTRIC LIGHTING. 335 



perfectly continuous, but it pulsates with the rise and fall of the 

 alternating current. Ordinarily these rapid pulsations of the light 

 are not perceptible unless one looks at a moving object which is 

 illuminated by the lamp. 



The two carbons of an alternating-current arc lamp waste away 

 at approximately the same rate ; the upper carbon, however, is 

 always consumed a little more rapidly than the lower carbon. 



The alternating-current arc makes a loud humming noise on 

 account of the repeated heating and cooling and consequent ex- 

 pansion and contraction of the column of conducting vapor. 



The alternating-current arc is much more unstable than the 

 direct-current arc, because the alternating current passes repeat- 

 edly through zero value and therefore the conducting column of 

 vapor cools slightly and the next pulse of current encounters at 

 first an increased resistance. Cored carbons are therefore gener- 

 ally used in alternating-current arc lamps, that is to say, carbons 

 which have a central core of soft carbon slightly impregnated 

 with metallic salts, the vapors of which are good electrical con- 

 ductors. The metallic vapors from cored carbons tend, however, 

 to form a coating on the bulb of enclosed arc lamps, and one 

 cored and one solid carbon are therefore generally used in en- 

 closed alternating-current arc lamps. 



The instability of the alternating-current arc is very great at low 

 frequencies on account of the longer time that the current is very 

 small before and after each reversal of current ; in fact, it is not 

 feasible to operate alternating-current arc lamps at frequencies 

 much lower than about 60 cycles per second. 



An important result of the increase of the resistance of the 

 alternating-current arc after each passage of the current through 

 zero is that the power in watts delivered to the arc is less than 

 El, where E is the electromotive force across the arc as measured 

 by an alternating-current voltmeter, and / is the current as meas- 

 ured by an alternating-current ammeter. In fact, the power is 

 equal to pEI, where /, which is less than unity, is what is called 

 the power factor of the arc. The value of / for the alternating- 



