330 Lord Rayleigh's Comparison of Methods for the 



the presumption is in favour of any method which requires 

 only a single linear measurement. It is true that this ques- 

 tion cannot be decided without regard to the subject of the 

 measurement ; but, with scarcely an exception, it is necessary 

 to know the mean radius of a coil of several layers of insulated 

 wire. This is apparently the measurement which fixes the 

 limit of final accuracy; and, in comparison with it, determi- 

 nations of the distances of mirrors and scales &c. are of 

 secondary difficulty. 



It will be convenient now to enumerate the principal 

 methods which have been proposed for determining absolute 

 resistances. Minor details, which are not likely to influence 

 the final value of the results, will in general be passed over. 



I. Kirchhoff's Method, Maxwell's Electricity and Magnetism, 

 § 759. 



The magnitude of a continuous battery-current in a pri- 

 mary coil- is compared with that of the transient current 

 induced in a secondary coil when the primary circuit is re- 

 moved. Rowland* effected an important improvement by 

 simply reversing the battery-current without motion of the 

 primary coil. The time of vibration of the ballistic galvano- 

 meter employed for the transient current is the principal time- 

 measurement. In Rowland's investigation a second galvano- 

 meter was employed for the battery-current, and the ratio of 

 constants had to be found by auxiliary experiments. In Glaze- 

 brook's f recent determination by this method only one galva- 

 nometer was used, the battery-current being reduced in a 

 known manner by shunting. It is shown that the evaluation 

 of the resistance-ratios presents no serious difficulty. 



Let h denote the ratio in which the primary current is re- 

 duced when it produces a deflection a upon the galvanometer, 

 6 the throw from rest due to the induction-current when the 

 battery is reversed, T the time of vibration of the needle mea- 

 sured from rest to rest, M the coefficient of induction : then 

 the resistance of the secondary circuit in absolute measure is 

 given by 



-r, ir tan «M 7 



Whenever, as in this method, the conductor whose resistance 

 in absolute measure is first determined is composed of copper, 

 frequent comparisons are necessary with standards of German 

 silver or platinum-silver. Otherwise a variation of tempera- 

 ture of about £ of a degree Cent., which can hardly be detected 



* American Journal, xv. 1878. 

 t Proc. Roy. Soc. June 1882. 



