ON ODM'S LAW. 



Now let u suppose two conductors of the same material but of different 

 A-~i~~ arranged in series and the same current passed through both : 



where the suffixes indicate to which conductor the quantities belong. The 

 ratio of the resistances is 



Hence if Ohm's law is not true, and if, therefore, any of the quantities s, s', 

 Ac. have sensible values, the ratio of the resistances will depend on the strength 

 of the current 



Now the ratio of two resistances may be measured with great accuracy 

 by means of Wheatstone's bridge. 



We therefore arrange the bridge so that one branch of the current passes 

 first through a very fine wire a few centimetres long, and then through a 

 much longer and thicker wire of about the same resistance. The other branch 

 of the current passes through two resistances, equal to each other, but much 

 greater than the other two, so that very little of the heating-effect of the 

 current is produced in these auxiliary resistances. 



The bridge is formed by connecting the electrodes of a galvanometer, one 

 to the junction of the fine wire and the thick one, and the other to a point 

 between the other two resistances. 



We have thus a method of testing the ratio of the resistances of the 

 fine wire to that of the thick one; and by passing through the bridge some- 

 times a feeble current and sometimes a powerful one, we might ascertain if 

 the ratio differed in the two cases. 



But this direct method is rendered useless by the fact that the current 

 generates heat, which raises the temperature of both wires, but that of the 

 thin wire most rapidly ; and this makes it impossible to compare the effects 

 of strong and weak currents through a conductor at one and the same 

 temperature. 



