ON OHM'S LAW. 537 



It is also useless to work with weak currents, as the effect depends on 

 the square of the current, and is so small as to have escaped observation in 

 all ordinary experiments. 



Again, if we were to use a single very strong current acting for a very 

 short time, we should not be able to observe the galvanometer in a satis- 

 factory manner. In fact it was found in the experiment that currents which 

 lasted for a sixtieth part of a second produced a heating-effect which interfered 

 with the measurements. The experiment was therefore arranged so that a 

 strong current and a weak one were passed through the bridge alternately ; 

 and when the bridge was so arranged that the galvanometer was in equili- 

 brium, the direction of the weaker current was reversed. If Ohm's law were 

 not true, the condition of equilibrium for strong currents would be different 

 from that for weaker ones, so that when the weak currents were reversed 

 there would be no longer equilibrium. Since, in point of fact, the reversal 

 of the weaker currents did not affect the equilibrium, it follows that the 

 bridge was in equilibrium for the weaker currents as well as for the stronger 

 ones, and therefore the conditions were the same for both, and Ohm's law 

 is true to within the limits of error of the experiment 4 '". 



[* The mode in which the actual strength of the currents was measured and the limits of 

 error ascertained are described in the Report which follows the above by Mr Chrystal, now Pro- 

 fessor of Mathematics at Edinburgh University.] 



VOL. II. 68 



