Resistance according to an absolute Standard. 233 



depends on the measurement of the magnitudes 



r, R, t, a; 

 in other words, the resistanee of the whole closed conductor can 

 be expressed in an absolute measure, if by observations, first, 

 the number a has been found which the deflection of the needle 



gives in parts of the diameter; secondly, the number ^-, which 



gives the diameter of both circles in parts of the distance BC ; 



thirdly, the velocity -, with which the diameter of those circles is 



traversed during one rotation of the needle. Hence it appears 

 that the measure of velocity is the only measure which must be 

 given if the resistance of a conductor is to be determined accord- 

 ing to an absolute standard. 



§ 3. Observations. 



Of the four magnitudes which, according to the preceding 

 paragraph, are to be found by observation for the purpose of 

 determining electric resistances according to an absolute stan- 

 dard, three can readily be measured, namely, the diameter r of 

 the two circles, the distance BC = R of the circle B from the 

 needle at C, and the time of oscillation of the needle t. There 

 only remains the fourth magnitude, that is the deflection of the 

 needle a. expressed in parts of the diameter, and this is usually 

 so small that it cannot be observed. This is the reason why, in 

 actually making the observations, a slight deviation must be 

 made from the arrangement described in the previous paragraph. 

 For in order to obtain a value of a large enough for accurate 

 observation, it is first necessary that the magnetic needle, upon 

 which the circular current B is to act, instead of being at a great 

 distance BC = R, be suspended in the centre of the circular cur- 

 rent itself, in which case the action is the greater the smaller is 

 the diameter r in comparison with R. Care must also be taken 

 that the length of the needle is much smaller than the diameter 

 of the circle, in order that the peculiar distribution of the mag- 

 netism in the needle need not be taken into account, because the 

 investigation of this distribution is attended with difficulties. It 

 is further necessary that both circles", instead of one, shall consist 

 of several windings of the conductor, by which they become 

 changed into rings of large diameter. In that case, however, the 

 influence of all the windings must be individually taken into 

 account, because they have different diameters, and are not all on 

 the same plane as the needle. 



For the conductor whose resistance was to be measured, a very 

 long thick copper wire was chosen which weighed 169 kilo- 



Phil. May. S. 4 Vol. 22. No. 146. Sept. 1861. R 



