230 M. W. Weber on the Measurement of Electric 



It is self-evident that a change of the given measures has no 



(r m m'\ 

 -g ) ; but a change of 



the given measures of time and space does influence the value of 



fi 

 the factor — , and accordingly the value of the number w, if both 



measures are not simultaneously increased or diminished in pro- 

 portion. The value of the number w is hence quite independent 

 of all alterations of the given measures, so long as there is no 

 change in the measure of velocity. But if, by an alteration of the 

 given measures, the standard of velocity is increased or dimi- 

 nished n times, an n times larger or smaller value is obtained for 



r' 

 the factor -, and therefore also for the number w, which is as 



much as to say that the resistance in this case is expressed 

 according to an n times smaller or larger standard. The un- 

 changeability of the unit of measure for resistance merely 

 depends therefore on the unchangeability of the given measure 

 of velocity. But if the measure of velocity is taken n times 

 larger or smaller, the unit of measure for resistance becomes 

 simultaneously n times larger or smaller. 



§ 2. Method of measuring Electric Resistance according to an 

 absolute standard. 



The measurements of length and of time, which, according to 

 the preceding paragraph, are adequate for the determination of 

 electric resistance, presuppose circumstances on the convenient 

 arrangement of which the practical execution and accuracy of 

 such a determination depend. The following arrangement may 

 serve as a simple summary of the essential circumstances. 



Out of the galvanic conductor whose resistance is to be deter- 

 mined, two circular rings, A and B, are formed, which are con- 

 nected in the manner represented in the * 

 figure. The whole conductor, consisting of ^-^ ^-^ (J 



the two circles A, B, and the junctions form ( \ J. \ 



a continuous line, of which it may be assumed, ^-^ 

 for the sake of simplicity, that it is situate in one plane, and that 

 the straight line connecting the centres of both circles coincides 

 with the direction of the earth's magnetism. Let T b'e the force of 

 the earth's magnetism as determined according to an absolute 

 standard by magnetometric measurements ; let r be the diameter 

 of the circles, which, for simplicity sake, are assumed to be equal. 

 If now the circle A is projected in the direction of the earth's mag- 

 netism AB on a plane normal to AB, the area of the projected 

 plane is 0. From the flexibility of the wires connecting the 



