Unit of Candle-power in White Light. 85 



The conclusion is that, it' work of the very highest accuracy 

 is to be carried out with flame-standards under abnormal 

 humidity conditions, the combined humidity-temperature 

 coefficient should be determined for the locality in which 

 the work is to be conducted. It should be pointed out that 

 the difference between the American and English determi- 

 nations for the pentane lamp amounts to less than 1 per cent. 

 in candle-power for a rise of humidity of 10 litres per cubic 

 metre above the normal. Values have seldom been observed 

 greater than this in Teddington. The table at the conclusion 

 of the paper by Crittenden and Taylor shows the average 

 humidities in Boston and New Orleans to be 9*9 and 19*1 

 litres per cubic metre respectively. Where such large 

 differences exist in climatic conditions the question of the 

 variation of the combined temperature-humidity coefficient 

 might with advantage be further investigated. 



(6) The Constancy of the Unit of Candle-power held in 

 Klectric Sub- standards. 



The continual use which has been made of electric sub- 

 standards since the first determination of the unit of candle- 

 power in 1904, has afforded opportunities of watching the 

 behaviour of such lamps and particularly of observing their 

 constancy. 



Several observers have written on this subject, amongst 

 whom are J. A. Fleming, C. H. Sharp, P. S. Millar, E. B. 

 Rosa, Gr. W. Middlekauff *, and others, and all witness to 

 the constancy of properly prepared and seasoned electric sub- 

 standards. Their observations have been mainly concerned 

 with carbon filament glow-lamps, and the present authors 

 are able to endorse the views they express. In order to 

 keep records of their behaviour, an annual analysis is made 

 of all the photometric records where sub-standards have 

 been used during the year. When an ordinary routine 

 standardization has to be made, three to six sub-standards 

 are put on the bench in turn, and thus for each day's work a 

 value is obtained for any one standard lamp in terms of 

 others of the same set. The analysis of such results over 

 a year's working indicates if any individual lamp shows 

 signs of differing from the mean of the others. It has been 



* Flemiug, Proc. Brit. Assoc. 1904 ; Sharp and Millar, Trans. 111. 

 Eng. !Soc, X.I., June 1910; Sharp, American Gas Inst., Oct. 1913; 

 Rosa and Middlekauff, Proc. Amer. Inst. Elect. Eng., July 1910, 

 p. 1011. 



