9 



NOTE ON KOHLKAUSCJFS DETERMINATION OF THE ABSO- 

 LUTE VALUE OF THE SIEMENS MERCURY UNIT OF 

 ELECTRICAL RESISTANCE 



[Philosophical Magazine [4], L, 161-163, 1875] 



In looking over Kohlrausch's paper 1 upon the determination of a 

 resistance in absolute measure, with a view to undertaking something 

 of the kind myself, and also, if possible, to discover the reason of the 

 difference from the results of the Committee of the British Association, 

 I think I have come across an error of sufficient magnitude and in the 

 proper direction to account 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 determinations of 

 the logarithmic decrement both with the circuit closed and open, to- 

 gether with various absolute determinations rendered necessary by this 

 change. In this way the logarithmic decrement is raised from being a 

 small correction to a most important factor in the equation. Hence 

 it is that we should carefully 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 divided 

 into two principal parts first, that due to the resistance of air and 

 viscosity of suspending fibre, and, second, that due to the induced cur- 

 rent in the coils. The first resistance is usually taken as proportional 

 to the velocity, and thus assumes the viscosity of the air to be the most 

 important element. This is proba,bly true in most cases where the 

 motion is slow. This factor is quite small compared with the second 

 when the magnet is large and heavy and the coils wound close to it, as 



^oggendorff's Annalen, Erganzungsband vi, p. 1; translated in Phil. Mag., S. 4, 

 vol. xlvii, pp. 294, 342. 



