THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 

 k 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



SEPTEMBER 1875. 



XX. Note on Kohlrausch's Determination of -the Absolute Value 

 of the Siemens Mercury Unit of Electrical Resistance. By 

 Henry A. Rowland, of the Johns Hopkins University, Balti- 

 more, Ma 1 .* 



IN looking over Kohlrauscb/s paper t upon the determination 

 of a resistance in absolute measure, with a view to under- 

 taking something of the kind myself, and also, if possible, to 

 discover the reason of the difference from the results of the Com- 

 mittee of the British Association, I think I have come across an 

 error of sufficient magnitude and in the proper direction to ac- 

 count for the 2-per-cent. difference. Kohlrausch's experiments 

 were made with such great care and by so experienced a person 

 that it is only after due thought and careful consideration that I 

 take it upon me to offer a few critical remarks. 



We observe, then, first of all, that the principal peculiarity of 

 his method consists in doing away with all measurements of the 

 coils of the galvanometer, and in its place making accurate de- 

 terminations of the logarithmic decrement both with the circuit 

 closed and open, together with various absolute determinations 

 rendered necessary by this change. In this way the logarithmic 

 pecrement is raised from being a small correction to a most im- 

 dortant factor in the equation. Hence it is that we should care- 

 fully scrutinize the theory and see whether it be correct enough 

 for this purpose ; for only an approximation is needed for the 

 first method. 



The resistances to a bar magnet swinging within a coil may be 



* Communicated by the Author. 



\ Po gendorff's Annalen, Erganzungsband vi. p. 1 ; translated in Phil. 

 Mag. S. 4. vol. xlvii. pp. 294, 342. 



Phil. Mag. S. 4. Vol. 50. No. 330. Sept. 1875. M 



