A NEW CURRENT WEIGHER, ETC. 535 



As numerous determinations of the balancing masses for (D + S) and (I) S) have 

 l>een made, the value of S for 1 ampere can be calculated from them with considerable 

 accuracy. By using this value a determination of current, using both sets of coils, 

 can be made by taking the apparent change of mass produced by a single reversal of 

 current in the fixed coila The necessary observations can lie made in less than five 

 minutes, so that a very short time would suffice for making an absolute determination 

 of current in this way. 



History of the Standard Cell employed. When the first determination of current 

 was made, the cadmium cell chosen for insertion in the potentiometer circuit was one 

 whose E.M.F. was lower than that of normal cells by O'll millivolt. Originally it 

 was not proposed to use this cell permanently, but as its previous history indicated it 

 to have remained very constant, it was afterwards decided to do so. The cell was 

 compared with other standard cells on each day that a determination of current was 

 made and on many other intermediate days. All the cells were constructed in the 

 manner descritad by one of us (F. E. S.) in the 'Report of the British Association,' 

 Section A, 1905, and were set up at the National Physical Laboratory. In the first few 

 determinations the cadmium cell was in the same room as the ampere balance, and its 

 temperature sometimes varied from 6C. to 19C. within 24 hours. Careful observa- 

 tions showed that the E.M.F. of the cell did not very closely follow this rapid change 

 in temperature, and the corrections to the value of CxR in Table XIII., Column 9, 

 were obtained from a curve which, though not very different from the temperature- 

 coefficient curve of the cell, is not identical with it. This statement applies to the 

 first twelve observations only, for on and after November 23, 1905, the cell was kept 

 in the resistance-standards room, which is maintained at a nearly constant tempera- 

 ture of 17C. After November 23, the correction to 17C. was obtained from the 

 temperature-coefficient formula 



E, = E 17 -3-4 6 xlO- 6 (*-17)-0-OGGxlO- 8 (<-17) 1 . 







This formula is the result of a determination made at the National Physical 

 Laboratory, the range of temperature during the observations being 10C. to 30 C. 

 The coefficients are practically identical with those given by JAEGER and KAHLE.* 

 Their formula is 



E ( = 1 '0186-0-000038 (<-20)-0'00000065 (<-20) J . 



The cell employed in the potentiometer circuit (hereafter called No. 2) was set up 

 in January, 1905 ; those with which it has been compared were set up on various 

 dates ranging from October, 1904, to April, 1907. The comparisons indicate that the 

 cells have remained constant within a few hundred-thousandths of a volt, or have 

 changed uniformly. The actual differences between the cells are not given here, but 

 may be summarised by saying that with the exception of cell No. 2 the greatest 



* 'Zeitechr. f. Instranentenk., 1 1898, p. 161. 



