128 REPORT — 1894. 



degree. But we are not concerned with the coefficient of mutual induc- 

 tion in this case. We are concerned with another integral, viz., 



J a. 



aUda 



and the adjustment for centre is in truth of primary importance. Special 

 attention should therefore be paid to this in designing apparatus for the 

 absolute measurement of resistance by this method. 



One other point remains to be noticed in this connection, viz., the pos- 

 sible effect of the difference of the temperature of the coil and disc when 

 measured and when in use. On calculating the correction to be applied 

 for this cause I find it negligible. 



Again, I would say, as I said last year, that the chief value of these 

 observations consists in the proof they afford of the precision with which 

 the absolute measurement of resistance may be made by this method. A 

 well-constructed apparatus of the kind in a national laboratory^say the 

 Laboratory of the Board of Trade — will, I believe, prove to be the best 

 ultimate standard of electrical resistance. 



APPENDIX III. 



Comparison of the Standard Coils used hy Professor Jones with 

 the Standards of the Association. By R. T. Glazebrook. 



The tenth-ohm standards of manganin wire whose value in absolute 

 measure was determined by Professor Jones by means of the experiments 

 described in Appendix II. were compared with the standards of the 

 Association in the following manner. A Wheatstone's bridge was formed 

 in which the arms were the tenth-ohm to be tested, two single-ohm coils, 

 and a ten-ohm coil ; if the coils had these values exactly, there would of 

 course always have been a balance ; since, however, the coils were not 

 accurately correct, there was usually a small current through the galva- 

 nometer ; the balance, however, could be obtained by placing a large re- 

 sistance as a shunt either to one of the one-ohm coils or to the ten-ohm 

 coil : this resistance, which varied from 10,000 to 20,000 ohms, was taken 

 from a good box of coils. The resistance of the ten-ohm and of the two one- 

 ohm coils being known, that of the tenth-ohm coil could readily be found. 



The four coils dipped into four mercury cups cut in an ebonite block ; 

 the bottoms of these cups were copper pieces some 3 to 4 mm. thick. 



Binding screws screwed into these copper pieces and rising above 

 the mercury served to connect the bridge to the galvanometer and the 

 battery. 



The mercury cups were somewhat large — about 2'5 cm. in diameter — 

 and it was found on January 16 that distinct differences could be observed 

 by moving the tenth-ohm coils slightly so as to bring their terminals 

 either close to or as far as possible from the feet of the one-ohm coils 

 which dipped into the same cups. After this date then two sets of 

 measurements were made for each coil at each observation : in the one 

 the terminals of the coils in any cup were put as close together as possible, 

 in the other the terminals of the tenth-ohm coils were placed at some distance 

 from those of the other coil in the same cup. 



Both sets of values are given in the table as a means of showing the 

 delicacy of the observations and the error aiising from this cause. The 



