344 ELEMENTS OF ELECTRICAL ENGINEERING. 



of graphitic carbon that is deposited upon the filament by the 

 flashing process. Near the working temperature the resistance 

 of the filament does not vary greatly with temperature and the 

 variation, such as it is, may be an increase or a decrease of resis- 

 tance with rise of temperature according to the amount of carbon 

 deposited on the filament by the flashing process. 



144. Relation between voltage, candle-power, efficiency and life 

 of carbon-filament glow lamps. The voltage of a lamp as 

 specified by the manufacturers is the electromotive force which it 

 is intended should be applied to the terminals of the lamp in 

 service. The effect on the lamp of increasing or decreasing this 

 electromotive force is considered in this article. 



Candle-power. Owing to the difficulty * of measuring the true 

 spherical-candle-power of a lamp it is customary to rate glow 

 lamps on their mean horizontal candle-power, that is, on the 

 candle-power which is indicated by the Bunsen photometer when 

 the lamp is rotated about a vertical axis at a speed of three or 

 four revolutions per second. Thus the curve in Fig. 175 repre- 

 sents the distribution of light around a glow lamp of which the 

 mean horizontal candle-power is 16 and of which the true spheri- 

 cal-candle-power is 13.33. The spherical -candle-power of a glow 

 lamp is usually from 80 to 85 per cent, of the mean-horizontal 

 candle-power. 



Efficiency. The efficiency of an electric lamp is usually speci- 

 fied by giving the watts consumed per candle-power, f The 

 usual efficiency ratings of carbon-filament glow lamps range 

 from 3. i to 3.5 watts per mean horizontal candle. High-efficiency 

 lamps (low watts per candle) should be used where the cost of 

 power is high, and low-efficiency lamps should be used where 

 power is cheap. 



* It is not difficult to measure true spherical-candle-power with a Matthews inte- 

 grating photometer ; all manufacturers and all large users of glow lamps should use 

 this photometer and base the ratings of their lamps on spherical-candle-power. 



f See discussion of lamp efficiency and the thermodynamics of radiation in Art. 



