ELECTRICAL UNITS OF MEASUREMENT. 89 



when to make the change, should a change be 

 necessary from the British Association unit as 

 the ohm, or from the Siemens unit, to bring 

 measurement into more close agreement with the 

 absolute reckoning. What had been done by 

 Lord Rayleigh and Mrs. Sidgwick had left very 

 little room for doubt but that the British Associa- 

 tion unit was in error to the extent of 1*3 per 

 cent. The Siemens unit had the advantage of 

 being somewhat approximately equal to the 

 desired absolute unit, though not professing to 

 be an absolute unit at all. It was simply the 

 resistance of a column of mercury at zero tem- 

 perature, a metre in length and a square millimetre 

 in section. There were great difficulties in the 

 reproduction of the Siemens unit, in the earlier 

 times of the investigation ; but Dr. Werner 

 Siemens, and Lord Rayleigh, and Mrs. Sidgwick, 

 and many other workers besides, all working to 

 compare with the British Association unit, obtained 

 results which finally left no doubt whatever as to 

 the true relation. Dr. Werner Siemens's result 

 found the mercury unit to be 0^95 36 of the 



