ELECTRICAL UNITS OF MEASUREMENT. 87 



portant practical question to decide. A proposal 

 had been before the world for ten years at least, to 

 found accurate measurement of electrical resistance 

 upon a material obtainable in uniform quality 

 and by easy precautions in a state of perfect 

 purity, or sufficiently nearly perfect to fit it 

 practically for the purpose in question, which 

 is, the giving of a standard for the measurement 

 of resistance. The Siemens unit, founded upon 

 the specific resistance of mercury, had been pro- 

 posed. The great house of Siemens (Berlin and 

 London), our distinguished confrere, Sir William 

 Siemens, and his distinguished brother, Dr. Werner 

 Siemens, worked upon this subject in the most 

 thorough and powerful way the measurement of 

 resistances in terms of the specific resistance of 

 mercury in such a manner as to give us a standard 

 which shall be reproducible at any time and place, 

 with no other instrument of measurement at hand 

 than the metre measure. I say, the system of 

 measurement of resistance on a mercury standard 

 had been worked out, and its practicability demon- 

 strated. Werner and William Siemens themselves 



