66 



Messrs. Paterson and Dudding on the 



pentane lamp, has the same coloured radiation as a black 

 body at 1610° C, whilst a tungsten lamp at 1*5 watts per 

 mean horizontal candle (line 6) matches a black body at 

 1950° C. Now, between these extremes there exists a con- 

 siderable colour difference, and this has to be bridged 



Fig. 1. 



t 



£ 20OC 



-C-- ■ 



J*-"th 





-=^*=~^9 





>a*.2:™ a 





-*£r 6 4_ 



^;iJ= == = = == = 



*£ -l*" 2 



-*2-0 



1 



1 -4- 



t 













18 



Ordinary Tungsten (Curve B) and Carbon (Curve A) Filament Vacuum 

 Lamps. Curves connecting lumens per watt * of the Lamps with, 

 the Temperature of a Black Body whose Radiation is identical in 

 Colour with that of the Lamps. 



photometrically when defining the candle-power of the 

 tungsten lamp in terms of the pentane unit. If, in order 

 to certify them, tungsten filament sub-standards were com- 

 pared on every occasion against the pentane lamp, or sub- 

 standards of the same colour, the large colour difference 

 would be a constant source of trouble and uncertainty. To 

 obviate this difficult}', a set of metal filament sub-standards 

 running at 1/5 watts per mean horizontal candle has been 

 standardized by the cascade method, and values of candle- 

 power in terms of the pentane unit have been assigned to 

 them once for all. The method used and results obtained in 

 standardizing these tungsten lamps are the principal subjects 

 of this paper. 



M S O P 



* Assuming a ratio of ' ' ' of 0*85 and 079 for carbon and 

 JM.hL.CJr. 



tungsten filaments respectively, the approximate watts per candle are 

 obtained for any value of lumens per watt by dividing the latter into 

 10*7 for carbon filament lamps, and 10'0 for tungsten filament lamps. 



