THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



MA Y 1866. 



XLIX. On the Question of the Unit of Electrical Resistance. 

 By Werner Stemens*. 



N the year 1860 I published f a method by means of which 

 I had succeeded in constructing exact resistance- standards. 

 I then proposed to accept, as unit of conducting-povver, that of 

 mercury at 0°C, and as unit of resistance that of a prism of 

 mercury one metre long and one square millimetre section, or 

 a million times the resistance of a cubic metre of mercury at 

 0°C. 



The grounds upon which I supported my proposition were 

 briefly as follows : — 



The adoption of an arbitrary material unit of electrical resist- 

 ance, or one more or less inseparable from some natural measure 

 to be set up somewhere like the normal metre and multiplied 

 by copying it, is not advisable, because we have not sufficient 

 guarantee of the electrical permanency of the materials; and 

 were we even sure of the permanency of such a standard mea- 

 sure, there could be no question that repeated copying and re- 

 copying in different materials, coupled with their possibly differ- 

 ent behaviours in regard to molecular changes, would soon end 

 in the distribution of faulty measures, as was the case, in so 

 marked a degree, in the copies of Jacobi's normal standard. 



The resistance-unit to be adopted must therefore consist of a 

 definition, or be an absolute measure which can be at any time 

 and in any place reconstructed. As such, Weber's dynamic 

 resistance-unit would be well qualified for scientific purposes, if 

 it could be reproduced with an exactness equal to what we 

 are entitled to look for in the comparison of two different 

 resistances. 



* Communicated by the Author. 



f Po":gendorff 's Annalen, vol. ex. p. 1 . [Phil. Mag. S. 4. vol. xxi. p. 25.] 

 Phil Mag. S. 4. Vol. 31. No. 210. May 1866. Z 



