392 COMPARISON OF RESISTANCES. 



the real resistance of the liquid between the electrodes, their 

 difference of potential is equal to 



H-H' + IR. 



The difference H - H' = e is what is called the electromotive 

 force of polarization. This difference is null when the electrodes 

 are identical, and goes on increasing to a maximum with the 

 quantity of electricity which passes (252). 



If the external electromotive force E is less than this maximum, 

 the conditions tend towards equilibrium, and the current should 

 completely cease; the current does not in fact disappear, but 

 remains very feeble, without there being apparent any decomposition 

 of the electrolyte ; the current is just sufficient to keep the polar- 

 ization constant, and to restore losses by diffusion.* If the 

 electromotive force is greater than the maximum of polarization, 

 gas bubbles are given off on the electrodes. 



The immediate effect of polarization of the electrodes is then 

 to diminish the intensity of the current, and produce an apparent 

 increase of resistance. This effect was formerly attributed to a 

 special resistance which electricity is supposed to experience in 

 passing from a solid to a liquid, or conversely, and which was 

 called resistance of transition. The presence on the electrodes of 

 non-conducting deposits, gas bubbles for instance, may in certain 

 cases introduce a new resistance which might be called resistance 

 of transition, but in a very different sense to the preceding. 



989. In order to get rid of polarization of the electrodes, 

 Wheatstonef worked with liquid columns of different lengths, and 

 kept the intensity constant by compensating variations of resistance 

 by means of a rheostate. For if we suppose the electromotive 

 force of the battery as well as the electromotive force e of polar- 

 ization to be constant, and call x, x the resistances of the 



liquid, p, p' the corresponding resistances of the rheostate, we have 



E-e E-e 



1 



R + * + p 



* HELMHOLTZ. Pogg. Ann., Vol. CL., p. 483. 1873. Wissenschaft. 

 AbhandL, Vol. I., p. 823. 



t WHEATSTONE. Phil. Trans., Vol. cxxxin., p. 303. 1843. Scientific 

 Papers, p. 122. 



