1865.] of Electrical Resistance. 159 



measure of resistance is styled n e - or . precisely as the corn- 



second second 



mon non-absolute unit of work involving the product of a weight into a 

 length is styled kilogrammetre or foot-pound. The Committee have 

 chosen as fundamental units the second of time, the metre, and the mass 

 of the Paris gramme. The metrical rather than the British system of 

 units was selected, in the hope that the new unit might 'so find better- 

 acceptance abroad, and with the feeling that while there is a possibility 

 that we may accept foreign measures, there is no chance that the Continent 

 will adopt ours. The unit of force is taken as the force capable of pro- 

 ducing in one second a velocity of one metre per second in the mass of a 

 Paris gramme, and the unit of work as that which would be done by the 

 above force acting through one metre of space. These points are very fully 

 explained in the British Association Report for ] 863, and in the Appendix C 

 to that Report by Professor J. Clerk Maxwell and the writer. 



The magnitude of the ^ is far too small to be practically convenient, 



and the Committee have therefore, while adopting the system, chosen as 

 their standard a decimal multiple 10 10 times as great as Weber's unit 

 / the millimetre\ Qr 1Q7 timeg ag ag ^ metre This m nitude 



\ second / second 



is not very different from Siemens' s mercury unit, which has been found 

 convenient in practice. It is about the twenty-fifth part of the mile of 

 No. 16 impure copper wire used as a standard by the Electric and Inter- 

 national Company, and about once and a half Jacobi's unit*. 



It was found necessary to undertake entirely fresh experiments in order 

 to determine the actual value of the abstract standard, and to express the 

 same in a material standard which might form the basis of sets of resistance- 

 coils to be used in the usual manner. These experiments, made during 

 two years with two distinct sets of apparatus by Professor J. C. Maxwell 

 and the writer, according to a plan devised by Professor W. Thomson, are 

 fully described in the Reports to the British Association for 1863 and 1864. 



The results of the two series of experiments made in the two years agree 

 within 0-2 per cent., and they show that the new standard does not pro- 

 bably diifer from true absolute measure by 0' 1 per cent f. It is not far from 

 the mean of a somewhat widely differing series of determinations by Weber. 



In order to avoid the inconvenience of a fluctuating standard, it is pro- 

 posed that the new standard shall not be called " absolute measure," or 



described as so many -- , but that it shall receive a distinctive name, 

 seconds 



such as the B. A. unit, or, as Mr. Latimer Clark suggests, the " Ohmad," so 



* This last number may be 30 per cent, wrong, as the writer has never been in pos- 

 session of an authenticated Jacobi standard, and has only arrived at a rough idea of its 

 value by a series of published values which afford an indirect comparison. 



t Vide Appendix B. 



