ON STANDARDS FOR USB IN ELECTRICAL MEASUREMENTS. 425 



as to either metliod of embodying the standard electromotive force, 

 thougli tboy are strongly inclined to believe that an electrometer or 

 cauo-e, capable of showing Avhen a definite electromotive force has been 

 developed, by whatever means, will ultimately be found more satisfactory 

 than any system in which the constancy of an electromotive force is 

 inferred from the supposed constancy of the conditions under which it 

 has been developed. 



Another question of a more general kind, v/hich, though it may not 

 be of much immediate practical importance, will eventually have to be 

 carefully considered, has occupied the attention of the Committee to 

 some extent. It is the question as to what concrete standards should 

 finally be recognised as fundamental standards. Supposing that we 

 already had independent standards of Ji^eslstance, Gapacltij, Electromotive 

 Force, Quantity, and Current Strength; each of them defined with all the 

 accuracy that our present experimental methods admit of, they would 

 infallibly be found to exhibit small discrepancies when compared together. 

 For instance, the current of standard strength would not be exactly the 

 same as that produced by the standard electromotive force acting in the 

 circuit of the standard resistance, and we should then have to consider 

 •which of the three standards was to be corrected so as to bring it into 

 harmony with the other two. 



Similarly, in the case of the other electrical magnitudes. The known 

 relations between these are sufficient to enable us to define the unit of any 

 one of the five magnitudes jnst mentioned in terms of the units of any 

 two of the rest. Hence it appears that the electrical standards of 

 ultimate authority cannot be more than two in number, and it will have 

 to be decided which pair of concrete standards are to be recognised as 

 ultimate or fundamental, and what are to be left to be defined by 

 reference to them. 



A further question arising out of the mutual relations of the funda- 

 mental units was that of the magnitude of the practical units to which 

 distinctive names should be attached. The present usage with respect to 

 this matter is that a resistance of 10^ C.G.S units is called an Ohm ; an 

 electromotive force oi \Q)*^ C.G.S. units is called a FbZi ; and the current 

 produced by a Volt acting through an Ohm, that is to say, a current of 

 10* -f- 10^ or 0-1 C.G.S. unit is called a Weher. In the opinion of the 

 Committee it -was considered highly desirable, from a scientific point of 

 view, that the relations among these standards should be simplified by 

 defining them as follows : — 



Ohm = 10^ C.G.S. units of resistance. 



Volt = 10^ C.G.S. units of electromotive force, 



Weber = 1 C.G.S. unit of current. 



It was felt, however, that any recommendation involving a change 

 in the value attached to terms which are rapidly coming into extensive 

 use among practical electricians, might give rise to serious inconvenience. 

 Therefore, although with regard to the scientific aspect of this proposal 

 the Committee were decidedly in favour of the change, they felt that a 

 public recommendation could not well be made until the practical incon- 

 veniences likely to follow had been very carefully investigated. 



