ELECTRICAL UNITS OF MEASUREMENT. 1 35 



determination is IO 9 X 0*98677, a difference of 2 

 in the 4th place, within about 1/50 per cent. 

 There is perfect magic in the accuracy of Joule's 

 work : it is not a matter of chance. I think, 

 between Joule, Lord Rayleigh, Mrs. Sidgwick, 

 and others, we cannot have much doubt now, 

 what is the absolute value of the Siemens unit, 

 or of the British Association unit. I advise 

 everybody to take the Rayleigh ohm unit, instead 

 of the British Association ohm. I have begun 

 to do so, and I mark everything R. O. You may 

 have everything in the British Association unit, 

 but reduce, if you please, to Rayleigh ohms by 

 the reducing factor 0*98677. Volts must be re- 

 duced in the same ratio. The old estimate which 

 I made in 1851 from Joule's experiment, for the 

 absolute electromotive force of a standard Daniell 

 cell, was 1*07 volts ; and after thinking it was 

 1*078 for ten years, because of the British As- 

 sociation unit, we come back to correct it, and 

 find it is 1*07. So much for the volt. But we 

 want far more accurate instruments and methods 

 connected with other parts of electric measure- 



