the Unit of Electrical Resistance. 329 



augmented that the difference between the two units is very 

 much greater than it appears. It is true that the determinations 

 of the constants are, for the most part, given as being exact 

 within 0000 i ; but we must conclude that this was only attained 

 in consequence of extraordinary manipulation and carefully se- 

 lected methods of measurement. The methods used are, how- 

 ever, not entirely free from sources of possible error. It is well 

 known, for example, that it is absolutely impossible to wind a 

 coil of silk-covered wire to an approximately round or solid coil 

 without stretching it. The amount of this stretching varies, with 

 the thickness and the tension of the wire (in winding), between 

 1 and 6 per cent. It would therefore be next to impossible to 

 conclude, with anything like certainty, within \ per cent., upon 

 the real length of the coiled wire. The real length of the 

 coiled wire, however, is given as 31 1*2356 metres. In addition 

 to this, it is quite impossible to wind such a coil perfectly round 

 and concentric with covered wire. The circumference, the mean 

 radius, the thickness of the coil, &c, can therefore only be given 

 approximately. But in the Reports the numbers representing 

 these values are given to within thousandths of a millimetre, 

 and are supposed to be trustworthy to within a ten-thousandth 

 part of their several values. There are other minor points by 

 which errors might creep into the observations, which would, 

 like the above, be constant for all the series made with the same 

 apparatus, and others which might affect individual observa- 

 tions. Whether the determinations of the magnetic moment 

 of the suspended needle and the horizontal component of the 

 earth's magnetism at the time of making the observation can 

 be carried to the same degree of accuracy, we must leave un- 

 decided. 



It is very far from my intention to assert that these measure- 

 ments were not in reality made with all the exactness which is 

 ascribed to them ; they can, however, in that case only be the 

 result of processes which have no general currency. 



Until these experiments have been repeated in other places, 

 with other apparatus, by other observers, and the result found 

 to agree with that of the Subcommittee, I think I am justified 

 in regarding the B. A. unit as representing 10 7 ™^ units only 

 to within some few per cent. 



For this reason I still hold to my objections against the adop- 

 tion of the material standards of the Subcommittee as the basis of 

 a general resistance measure. In doing so, 1 do not in the least 

 degree ignore the immense value of the experimental researches 

 which the different members of the Subcommittee have contri- 

 buted towards the solution of the questions at issue, and of the 

 determination of Weber's unit, which, under the auspices of the 



