264 M. W. Weber on the Measurement of Electric 



Kirchhoff's copper wire is compared with the resistance of 

 Jacobi's standard. I have made that comparison by means of 

 the wire which Kirchhoff kindly sent to me, and can here give 

 the result of the comparison : it is as follows. 



A piece of KirchhofPs wire which was 13*573 Prussian inches 

 in length and 0*4061 square line in section, had a resistance 

 which was to the resistance of Jacobi's standard as 



1 : 106. 

 From this we get the relation of the resistance of the measure 

 chosen by Kirchhoff to that of Jacobi's standard as 



1: 106 x 18-578 Xg^j- 



If J be the resistance of Jacobi's standard, and W that of Kirch- 

 hoff' s, we have 



1=510180. 



Now the resistance of Jacobi's standard is equal to 5980 million 

 units of the absolute measure found above ; hence, if W be the 

 absolute resistance, 



1=5980000000; 



hence 



W 



— =11720. 



W 



But now 



5! =26154; 



hence 



'CW'"224' 



that is, one-seventh less than Kirchhoff had found. A closer 

 agreement was not to be expected, inasmuch as only an approxi* 

 mate value was claimed for Kirchhoff' s statement. 



I may give here a determination of the specific resistance of 

 the different kinds of copper which have been used for Jacobi's 

 standard, for Kirchhoff's wire, and for the damper which I 

 used. 



The specific resistance of a body is usually given according to 

 an absolute unit by taking for this unit the specific resistance of 

 a body whose absolute resistance with a length = 1 and a 

 section =1 is equal to the fixed measure of resistance. But the 

 determination of specific resistance according to this unit meets 

 with a practical difficulty in the accurate measurement of the 



