356 



DR. J. A. BARKER ON THE HIGH-TEMPERATURE STANDARDS 



COIL Values in ohms at 17 C. 

 Coils in main row nominal value '1 each. 



Mean value of the 20 tenth-ohm coils = '100024. 



Total ,, ,, slide-wire set of 11 coils shunted by slide-wire = '100043. 



Maximum variation in the resistance of slide-wire set for different positions of the 

 slide-wire connector '000008. 



Value of nominal 100-ohm coil = 100'029. 



The auxiliary set of '01 ohm coils for temperature compensation were all found to 

 be within J-^QQ part of their face value. 



XII. Standard of Electromotive Force. 



The standards of E.M.F. used with this potentiometer were two similar H-form 

 cadmium-sulphate cells with saturated solution of the type employed at the 

 Reichsanstalt, and made up as part of a large batch of similar ones by Mr. F. E. SMITH 

 in 1902. From Mr. SMITH'S measurements as to the relation of the E.M.F. of these 

 cells to the standard Clark cells of the laboratory, and from other data, it is practically 

 certain that the error committed in assuming their E.M.F. to be identical with those 

 at the Reichsanstalt is not greater than 1 part in 10,000, which corresponds to a 

 tenth of a degree at 1000 C. with the thermoj unctions employed. For the E.M.F. 

 of each of these cells, which throughout the work were never found to differ by more 

 than '0001 volt, the Reichsanstalt official value, namely 1'0186 volt at 20 C., was 

 assumed. 



The following table gives the value of the total resistance used in the potentiometer 

 at different temperatures to adjust the E.M.F. over each coil of the main row to 

 exactly 1000 microvolts: 



The table of the values of the coils in the potentiometer shows that the actual 

 resistance corresponding to a nominal value of 101 '86 ohm is 101 '8 8 9. The relation 

 of this to the mean value of the main set of tenths, namely '100024 ohm, is well 



