80 Messrs. Paterson ami Duddins: on the 



n 



4. The Redetermination of the Constants of the 

 Pentane Lamp. 



The second realization of the unit of candle-power from 

 the pentane lamp, referred to on p. 72, has afforded an 

 opportunity of re-examining the constants of the pentane 

 lamp in regard to changes of humidity and barometric 

 pressure. As in the previous determination *, the compari- 

 sons extended over two summers and winters, and by this 

 means as wide a range as possible of humidity was secured. 

 The conditions for the use of the lamp were similar to those 

 existing previously, except that a larger number of observers 

 took part in the measurements, and there was no colour 

 difference between the light from the pentane lamp and that 

 from the sub-standards compared against it. An Assmann 

 ventilated hygrometer was used in the tests, and all humidities 

 are expressed in terms of its indications. In view of the 

 similarity of the experiments to those which have been 

 already described, it is not necessary to give the actual 

 observations in detail here. They have, however, been care- 

 fully analysed in the usual way by the method of least 

 squares, and the constants for variation of humidity and for 

 barometric changes are compared below with those previously 

 obtained. 



If C.P. = the candle-power of the pentane lamp, c = the 

 humidity in litres of water- vapour per cubic metre of moist 

 aii-, b = barometric pressure in millimetres, then 



(11)04-06 determination) ( 'I.P. = 10{ 1 + 0'0066(8-<?) 



-0*0008 (760-6)} . . (1) 



(1912-14 determination) C.P. = 10{ 1 + 0*0063(8 -<?) 



~0-0008 5 (760-/>)} . . (2) 



These two results may be regarded as practically identical, 

 and are supported by those obtained by Messrs. Sutterfield, 

 Haldane, and Trotter f, with whose formula the second one 

 given above is identical. 



A number of different determinations by various observers 

 have now been made of the constants of the pentane lamp 

 for changes of humidity and atmospheric pressure, and these 

 are scheduled in the following table : — 



* C. C. Paterson, " Investigations on Light Standards," Journ. I. E. E. 

 vol. xxxviii. p. 274. 



t Buttertield, Haldane, and Trotter, 'Journ. Gas Lighting/ 115, 

 p. 290 (1911). 



