10 Lord Rayleigh on Prof. Himstedt's 



It is eminently desirable, in the face of these facts, that 

 the imperfect formula of Lenz and Jacobi, as well as that of 

 Miiller, should henceforth disappear from the text-books of 

 physics, and that instead thereof the true law of the electro- 

 magnet should be stated, either in the exponential form or in 

 the simpler form that is found to be equally true within the 

 range of saturation attained in practice. 



II. On Prof. Himstedt's Determination of the Ohm. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



AS there is still some discrepancy in the values of the ohm 

 obtained by able workers using various methods, it 

 seems desirable to put forward any criticisms that may suggest 

 themselves, in the hope that the causes of disturbance may 

 thus come to be better understood. I propose accordingly to 

 make a few remarks upon the paper of Professor Himstedt, 

 translated in your November number, not at all implying that 

 his results may not be as good as any other, but rather in 

 order to raise discussion on certain points which the author 

 may be able to treat satisfactorily when he publishes a more 

 detailed account of his work. 



The leading feature in the method of Prof. Himstedt is the 

 use of a commutator, or separator, by which the make- and 

 break-induced currents are dissociated, one or the other passing 

 in a stream at equal small intervals of time through a galva- 

 nometer, by whose aid their magnitude is appreciated. The 

 instrument works with mercury contacts. When I first con- 

 sidered the methods available for the solution of this problem 

 at Cambridge in 1880, 1 found ready to my hand an ingenious 

 apparatus, contrived by Prof. Chrystall for this very purpose. 

 The contacts were effected by metallic dippers, controlled by 

 eccentrics, and passing in and out of mercury cups. What de- 

 termined me against this method*, notwithstanding its obvious 

 advantages in respect of sensitiveness, was the recollection 

 of unavailing attempts of my own in 1870 to make satisfac- 

 tory mercury contacts with dippers carried by electrically 

 maintained tuning-forks. Even when silver was the metal 

 employed, the contacts were uncertain, and no trustworthy 

 galvanometer deflection could be obtained. It may be men- 



* I may remark that Brillouin used a commutator of this nature in 

 his researches on the comparison of coefficients of induction : Theses 

 presentees a la Faculte des Sciences de Paris, 1882. 



