164 Mr. F. Jenkin on the Question of the 



diameter ? what Englishman, using a foot, thinks of pendulums ? 

 For practical use the material standard, not the definition, is the 

 important point. 



Further than this, the apparently simple definition might lead 

 to gross errors. It requires all Dr. Siemens' s skill to produce a 

 mercury unit, if even he can do it. The attempt on the part of 

 any ordinary optician to produce a mercury unit would result in 

 gross discrepancies, which unfortunately might not be discovered 

 until coils had been in use for years. The Committee, therefore, 

 did not think the difficulty of explaining the definition of their 

 unit of any serious importance. 



Hitherto I have endeavoured to show that there is no reason 

 to adopt Dr. Siemens' s definition more than any other ; I have 

 now to show why any other definition should have been preferred 

 to Dr. Siemens' s by the Committee. 



First to be considered were the practical units — the foot of 

 copper wire, or the mile of iron wire, &c. One objection to 

 these is, that to make them exact is to make them unpractical. 

 Pure metals or specific alloys must have been adopted, and in 

 practice these are never found. Practically no two feet of cop- 

 per wire or miles of iron wire are alike. There are other argu- 

 ments on this point which will be found in previous Reports. 



I now come to the definition adopted. The reason of this 

 adoption was that the absolute or natural units are those which 

 must necessarily be used in all mathematical calculations of the 

 relations between currents and magnets, according to the exist- 

 ing system of the measurement of magnets. They are, further, 

 most convenient in expressing the relations between all electrical 

 magnitudes themselves. How this is done I have already ex- 

 plained very fully in the Reports which, as Secretary, I had the 

 honour to write for the Committee, and which I am happy to 

 find meet with some approval by Dr. Siemens. Weber, Thom- 

 son, and Clerk Maxwell's writings are full of examples : the ab- 

 solute unit is as natural an expression of electrical resistance as 

 the cubic metre is of capacity when the lineal metre is the unit 

 of length. It is the unit which necessarily expresses resistance, 

 just as the cube necessarily expresses capacity. It would as 

 certainly have been used in all scientific investigations (even if 

 Dr. Siemens's unit had been in universal practical use) as the cubic 

 metre or cubic foot are used in calculation even where the practical 

 measures in use may be the chopin or the gallon. The Com- 

 mittee, therefore, considered that they had no choice in the mat- 

 ter so far as definition was concerned. On the one hand there 

 was a number of arbitrary definitions, on the other the natural 

 unit inevitably used in calculations : could their decision be 

 doubtful? 



