334 Lord Rayleigh's Comparison of Methods for the 



same magnet, first at one of these places, and then at the 

 other." 



If a be the mean radius of the coil of the inductor and A 

 that of the galvanometer, we may write, neglecting the cor- 

 rections for the finite sizes of the sections, 



9 /-< '2.1T 



so that 



J A 



This is the linear quantity of the method. With respect to 

 the chances of error in determining it, we see that the error 

 of the mean radius of the inductor enters doubly, and that of 

 the mean radius of the galvanometer enters singly. Probably 

 in this respect there is not much to choose between this method 

 and the use in method I. of the same coils placed at a mode- 

 rate distance apart. 



A colossal apparatus for the use of the present method has 

 been constructed and tested by MM. W.Weber and F. Zollner*, 

 the coils of which are as much as 1 metre in diameter. The 

 principal difficulty arises in connexion with the galvanometer- 

 magnet. Two magnets were used whose lengths were respec- 

 tively 200 millim. and 100 millim.; and the results obtained 

 in the two cases differed by as much as 2 per cent. The dis- 

 crepancy is doubtless due to the influence of the finite length 

 of the magnets causing the magnetic poles to be sensibly dis- 

 tant from the centre of the coil, for which point the effects are 

 calculated ; and the disturbance will be proportional to the 

 square of the distance between the poles, or more properly to 

 the " radius of gyration " of the ideal magnetic matter about 

 the axis of rotation. But to assume that the disturbance from 

 this source was exactly four times as great in the one case as 

 in the other, and thence to deduce the result corresponding to 

 an infinitely short magnet, appears to me to be a procedure 

 scarcely consistent with the degree of* accuracy aimed at. If 

 this method is to give results capable of competing with those 

 obtainable in other ways, it will be necessary to use a much 

 shorter magnet; or, if that is not practicable, to devise some 

 method by which the distance of the poles can be determined 

 and a suitable correction calculated. 



In carrying out the observations in the usual manner, it is 

 necessary to measure the distance between a mirror and a 

 scale. By using a double mirror with two scales and tele- 

 scopes, MM. Weber and Zollner avoid the principal cause of 

 * Ber. d, KiJn. Sactis. Ges. zu Leipzig, 1880, ii. p. 77. 



