Determining the Mechanical Equivalent of Heat, 171 



the value 1*434 as the electromotive force in volts of a Clark 

 cell at 15° C." 



These decisions agreed with those which had already been 

 laid before the German Government by Dr. Helmholtz and 

 the Curatorium of the Eeichsanstalt. It was considered pre- 

 ferable to define the mercury column in terms of length and 

 mass, as the sectional area cannot be directly measured with 

 such accuracy; but the length 106*3 centimetres was adopted 

 so that the sectional area might be, as nearly as could be 

 known, one square millimetre, as this area has been specified 

 in all previous definitions. 



In November 1892 a supplementary report was presented 

 to the Board of Trade by its Standards Committee, and in- 

 cluded the following resolutions : — 



" 4. That the resistance offered to an unvarying electric 

 current by a column of mercury at the temperature of melting- 

 ice 14*4521 grammes in mass of a constant cross sectional 

 area, and of a length of 106*3 centimetres, may be adopted as 

 one ohm." 



"10. That an unvarying current which, when passed through 

 a solution of nitrate of silver in water, in accordance with the 

 specification attached to this Report, deposits silver at the 

 rate of 0*001118 of a gramme per second, may be taken as a 

 current of one ampere." 



"14. That the electrical pressure at a temperature of 15° 

 Centigrade between the poles or electrodes of the voltaic cell 

 known as Clark's cell, prepared in accordance with the speci- 

 fication attached to this Report, may be taken as not differing 

 from a pressure of 1*434 volts by more than one part in one 

 thousand." 



At the International Electrical Congress held in August 

 1893 at Chicago these values were adopted by the delegates 

 as those to be recommended to their several governments as 

 legal units. 



The Board of Trade Committee, after having had laid before 

 them the recommendation of the Electrical Congress, which 

 was substantially that contained in their supplementary report 

 of November 1892, presented their final report advising a 

 legalization of the standards as defined, by an Order of Her 

 Majesty in Council ; and on August 23rd, 1894, this was 

 effected, and the practical standards we may hope permanently 

 settled. 



Of the three units — ohm, ampere, and volt — the two former 

 are defined on the C.G.S. system, and the latter as a secondary 

 unit from the relation of the first two. The numbers given 

 in the above-quoted resolutions as to the ohm and the ampere 



