346 REPORT — 1864. 



from self-induction, torsion of fibre, &c. For these corrections it is further 

 necessary to measure — 



1st. The coefficient of torsion of the fibre. 



2ncl. The magnetic moment of the sus|)en(ied magnet. 



3rd. The horizontal component of the earth's magnetism. 



4th. The variation of the electrical resistance of the coil during each ex- 

 periment and between each experiment. 



5th. The variation in the dii'ection of the earth's magnetic force. 



6th. The irregularities resulting from the unavoidable departures from 

 that relative position of the telescope, mirror, scale, and magnet which would 

 be theoretically most desirable. 



In the experiments made at King's College in 1864, every part of the 

 apparatus, except the distance of the mean planes of the two coils from the 

 axis of rotation, was altered ; so that every measurement was not only made 

 afresh, but, where susceptible of change, was considerably different in mag- 

 nitude. 



Pew of the measirrements could be made by the means employed with 

 greater accuracy than one part in 10000, and some of them were not deter- 

 mined even with this degree of accuracy. No very perfect agreement be- 

 tween two entii'ely distinct series of experiments was therefore to be expected ; 

 but the Sub-Committee, consisting of Professor Maxwell and Mr. Jenkin, 

 who this year have undertaken the experiments, are fortunately able to 

 report a concordance between the determinations of 1863 and 1804 which is 

 most satisfactory. 



The difference between a standard constructed from the mean result of the 



1863 experiments and a standard constructed from the mean resvdt of the 



1864 experiments would be only 0*16 per cent. The probable error of the 

 1863 experiments is 0*24 per cent, if the mean of each day's experiments 

 be counted as one only; the probable error of the 1864 experiments is O'l 

 per cent, if the mean of each pau* of experiments with the coU revolving in 

 two opposite dii'ections be taken as one experiment. 



Taking into account the agreement between the means of the two years, 

 we may say that the determination of the Sub-Committee does not probably 

 differ from true absolute measurement by 0-08 per cent. 



The Committee are of opinion that, in the present state of electrical science, 

 the result now obtained is satisfactory, and wiU justify the immediate con- 

 struction of final standards of electrical resistance. 



It can hardly be doubted that, ■wdth the lapse of time and the inevitable 

 progress of knowledge, stUl better determinations will some day be made ; 

 and that even now, with still greater care and by still further multiplpng 

 the number of experiments, a somewhat more perfect agreement between the 

 standards and the theoretical absolute measurement could be ensured. 



The Committee had then to consider whether this possibly still more per- 

 fect agreement would be wortli the veiy great time, the labour, and the 

 money which would have to be bestowed xipon it. It has never been pro- 

 posed that the British Association standard should be considered as repre- 

 senting exactly an absolute measurement ; whatever may be the state of 

 science, any such pretension could not be well founded, for all that can be 

 done at any time, by the very greatest care, is to reduce the possible error to 

 less than a certain amount. The amount of probable error in the present de- 

 termination is so smaU as to be insignificant for any of the present purposes of 



