Resistance according to an absolute Standard. 227 



fundamental measure for specific resistances would be necessary, 

 the introduction of which would be open to question. First, 

 because there would be no saving in the number of the funda- 

 mental measures if, in order to do without a fundamental mea- 

 sure for the absolute resistance, another fundamental measure 

 must be introduced which is otherwise superfluous. And secondly, 

 neither copper nor any other metal is fitted for use in establish- 

 ing a fundamental measure for resistances. Jacobi says that 

 there are differences in the resistances of even the chemically 

 purest metals, which cannot be explained by a difference in the 

 dimensions ; and that, accordingly, if one physicist referred his 

 rheostat and multiplicator to copper wire a metre in length and 

 1 millimetre thick, other physicists could not be sure that his 

 copper wire and theirs had the same coefficient of resistance, that 

 is, whether the sp)ecific resistance of all these wires was the same. 

 The reduction of measurements of galvanic resistances to an ab- 

 solute measure can therefore only have an essential importance, 

 and find a practical application, if it takes place in the first men- 

 tioned way, in which no other measures are presupposed than 

 those for electromotive force and for intensity. 



The question then arises, as to what are the measurements of 

 electromotive forces and intensities ? In measuring these magni- 

 tudes, no specific fundamental measures are requisite, but they 

 can be referred to absolute measure if the magnetic measures for 

 bar magnetism and terrestrial magnetism, as well as measure of 

 space and time, are given. 



As an absolute unit of measure of electromotive force, may be 

 understood that electromotive force which the unit of measure of the 

 earth's magnetism exerts upon a closed conductor, if the latter is 

 so turned that the area of its projection on a plane normal to the 

 direction of the earth's magnetism increases or decreases during the 

 unit of time by the unit of surface. As an absolute unit of inten- 

 sity, can be understood the intensity of that current which, when 

 it circulates through a plane of the magnitude of the unit of mea- 

 sure, exercises, according to electro -magnetic laws, the same action 

 at a distance as a bar-magnet which contains the unit of measure of 

 bar magnetism. The absolute measures of bar magnetism and of 

 terrestrial magnetism are known from the treatise of Gauss, 

 " Intensitas Vis Magneticse Terrestris ad mensuram absolutam 

 revocata," Gottingse, 1833 (Poggendorff's Annalen, vol. xxviii. 

 pp. 241 and 591). 



From this statement it is clear that the measures of electric 

 resistances can be referred to an absolute standard, provided mea- 

 sures of space, time, and mass are given as fundamental measures ; 

 for the absolute measures of bar magnetism and of terrestrial mag- 

 netism depend simply on these three fundamental measures. A 



