On the Measurement ofEtecfric Resistance. 261 



mercury a lens which flattens with a well-defined edge ; then a 

 rapid motion sets in from the edge and spreads all over the sur- 

 face ; bosses form and disappear ; the film becomes divided into 

 two or three parts by lines, but without separating ; the agitation 

 ceases ; the film spreads more and more -, but at a certain point 

 cohesion begins to reassert itself, and the film gradually contracts 

 and at length becomes a perfect circular disc. 



King's College, Loudon, 

 26th July, 1861. 



XXXIV. On the Measurement of Electric Resistance according to 

 an absolute Standard. By Wilhelm Weber. 

 [Concluded from p. 240.] 

 § 5. Comparison of the Resistance determined according to abso- 

 lute measure with Jacobi' s Standard of Resistance. 

 TO compare the resistance of two conductors, there are differ- 

 ent methods which need no explanation. The resistances 

 considered in the preceding paragraphs have been compared 

 according to the method examined in this memoir, and it has 

 been found that 



to :V=1138-! 1000. 



If the first resistance be reduced to the second according to this 

 proportion, we obtain 



w'=™|^=1903-10 8 , 



while the direct determination in the preceding paragraph gave 



w'=1898-10 3 . 

 From both these closely agreeing values, determined according 

 to entirely different methods, the number 19*10 8 will in future 

 be assumed as the mean value of this resistance. 



Jacobi has dwelt on the importance of introducing a definite 

 measure for resistance to be accepted by all physicists, especially 

 at the present time, when so many voltaic investigations are being 

 made with the most varied instruments, the comparison of whichis 

 often of great importance. For this purpose he has proposed as 

 a standard measure a copper wire, which he has sent to several 

 physicists who are engaged with voltaic measurements, and has 

 requested them to compare this standard with theirs, and for 

 the future to give their measurements in this measure. 



This standard is a copper wire 7169f millims. in length, and 

 | millim. in thickness, which weighs 22449 T 5 o milligrammes. 



The standard introduced by Jacobi, which, it is to be hoped, 

 will find general acceptance, is by no means supplanted by the 



