PHOTOMETRY AND ELECTRIC LIGHTING. 345 



Life. The carbon filament of a glow lamp deteriorates in ser- 

 vice, on account of what seems to be a vaporization of the carbon, 

 and the candle-power and efficiency decrease. The falling ofT of 

 candle-power is due in part to the fact that as the filament grows 

 smaller its resistance increases, the power delivered to it at the 

 standard voltage is reduced, and the working temperature is 

 lowered so that the light tends to become red instead of a yel- 

 lowish-white, and it is due in part to the blackening of the bulb 

 by the condensation of the carbon vapor. The falling off in effi- 

 ciency is due partly to the reduced working temperature of the 

 filament and partly to the absorption of light by the blackened 

 bulb. 



A glow lamp frequently becomes very dim and inefficient before 

 the filament actually breaks, and it is usually advisable to discard 

 a lamp and replace it by a new one when the candle-power has 

 fallen to a certain fraction of its initial value. The useful life of a 

 glow lamp is usually reckoned as the number of hours of actual 

 service that the lamp will stand before its candle-power drops to 

 80 per cent, of its initial value. 



The changes of candle-power and efficiency of a glow lamp 

 with the change of voltage between its terminals depend to some 

 extent upon the nature of the filament (ratio of amount of carbon 

 deposited by the flashing process to the amount of carbon in the 

 filament before flashing) ; and the change of life with the change 

 of voltage depends upon the size of the filament (low candle- 

 power with given voltage, and high voltage with given candle- 

 power, watts per candle being constant, mean a fine filament). 

 However, the following table may be used to estimate the ap- 

 proximate variation of candle-power, efficiency, and life of any 

 ordinary carbon-filament glow lamp. This table is based on the 

 i6-candle-power, 100- to 125-volt lamp taking 3.1 watts per 

 candle, and giving 14.7 average candle-power during its useful 

 life of 510 hours. 



It is to be noted that the basing of this table on the 3.1 watt 

 lamp is largely a matter of convenience. In fact the table gives 



