Resistance according to an absolute Standard. 263 



tion-constant e is left undetermined within the limits of that 

 variability of the resistance of copper. Kirchhoff himself says, 

 " Since the conductibility of copper varies within certain limits, 

 in giving the value of e, only a limited accuracy is of interest." 

 Kirchhoff wished to give only an approximate value of e, which 

 would be sufficient for his purpose ; and he was the more content 

 therewith because the methods and instruments which he used 

 would scarcely have permitted a better determination of e if he 

 proposed a perfectly definite measure of resistance. 



The interest which an accurate determination of the value e 

 has, is lost in consequence of that uncertainty in the choice of 

 the measure of resistance ; and it is important to restore it by the 

 removal of that uncertainty. This may be accomplished by 

 keeping, not to copper in general, but to the piece of copper 

 actually used by Kirchhoff in his investigations, and by choosing 

 the resistance of a wire of this copper 26'154 millims. in length, 

 and with a section of 684 square millims. as a measure of resist- 

 ance. It is thus only necessary to reduce the result found by 

 Kirchhoff, as well as the measures made therewith or referred 

 thereto, to the measure thus accurately determined in this manner, 

 Kirchhoff took one Prussian inch in a second as a measure of 

 velocity, and found in this way 



1 



6 ~192 ; 



from which it follows (since C' = 26 , 154 C) that that resistance 

 which amounts to 52*308 units of the above absolute measure 



is the =-q^- of the resistance of a wire of KirchhofPs copper the 



length of which is 26" 154 millims. and the section 584 square 

 millims. : in other words, that the measure of resistance chosen 

 by Kirchboff is 10043 times that of the above absolute measure. 



Although this value of e can only be considered as approxima- 

 tive, it is interesting to compare it with other values which have 

 been found by entirely different methods and with different in- 

 struments, because an examination of the various natural laws 

 brought thereby into operation is obtained. KirchhofFs mea- 

 surements refer to currents produced by voltaic induction, and 

 hence in his case it is the laws of voltaic induction which have 

 been used in determining the value of e. My measurements, on 

 the contrary, refer to currents produced by magnetic induction, 

 and hence in this case it is the laws of magnetic induction which 

 lead to the value of e. 



First of all, the value of e shall be given which is obtained 

 from my measurements. It is clear that the value of e can be 

 determined from these measurements, if only the resistance of 



