536 PROFESSOR W. E. AYRTON, MR. T. MATHER AND MR F. E. SMITH: 



difference in E.M.F. of any cell from the mean E.M.F. of all of them is 0'03 millivolt, 

 and the difference between the mean E.M.F. of the old cells and the new cells set up 

 in March and April, 1907, is 0'02 millivolt. This comparison indicates constancy of 

 the old cells ; Table XIV. confirms this view. The mean difference of the " old and 

 new cells" and cell No. 2 is O'll millivolt. On September 13, 1906, and on April 10 

 and 11, 1907, a cell representing the mean normal cell was employed in the deter- 

 mination of current. The results are contained in Table XIV. 



The mean value of CxR at 17 C. is 1'01830; the value from comparison with cell 

 No. 2 is 1'01830 7 . Both these values assume g to be 981 '20; correcting for the 

 difference of this and the accepted value 98T19, we obtain 



' T01830 " "":. 



as the mean value of CxR at 17 C. 



It should be pointed out that the " international ohm " used in these measure- 

 ments is that employed at the National Physical Laboratory, which unit does 

 not differ by more than 3 parts in 100,000 from that of the Reichsanstalt. In 

 absolute measure, however, its value is not known to a high degree of accuracy. 

 Taking its ratio to the Board of Trade ohm as determined by one of us (F. E. S.) in 

 1903 ('B.A. Report,' 1903, and 'Phil. Trans.,' A, voL 204) as I international 

 ohm = r0001 B B.O.T. ohm, and assuming that the B.O.T. unit has remained constant 

 since 1897, when its value in C.G.S. units was found to be 1 '00026* x 10 9 , we get 

 1 international ohm = r00041xlO* C.G.S. units, and the E.M.F. of the normal 

 cadmium cell at 17C. becomes 



\ 



T0187, x 10 8 C.G.S. units (approximately). 



This number must, however, be considered as provisional only, pending a re- 

 determination of the international ohm in absolute measure. 



It is of interdst to compare our value of C x R in terms of the international ohm 

 with that obtained by GUTHE in 1906.t He gives the number 1 '01 8 53 as the E.M.F. 

 at 20 C. of the E, K, and O series of cells set up with electrolytically prepared paste, 

 which cells are comparable with the " normal " cell used in our determination. 

 Allowing for difference of temperature, our value of CxR at 20 C. becomes 1 '01 819, 

 a difference of 34 parts in 100,000. 



As regards the Clark cell, the mean of a number of comparisons made at the 

 National Physical Laboratory gives the ratio 



Clark at 15C. -r- Cadmium at 17C. = 1'406 6 , 



* " On a Determination of the Ohm, &c.," by Professor W. E. AYRTON, F.R.S., and Professor 

 J. V. JONES, F.R.S. ' B.A. Report,' 1897. 



t " A New Determination of the Electromotive Force of WESTON and CLARK'S Standard Cells by an 

 Absolute Electrodynamometer," ' United States Bulletin,' vol. 2, p. 69. 



