248 ENERGY OF CURRENTS. 



same current, it may be asked whether it is the same weight of metal, 

 or the same weight of oxygen, which is liberated in the two electro- 

 lytes. M. E. Becquerel showed that the metalloid determines the 

 law -, consequently the weights of iron for the two electrolytes will 

 be in the ratio of 3 : 2. This is also the case with the salts of other 

 acids, the chlorides, sulphurets, etc. 



259. ELECTRICAL COUPLES. Let us now consider a compound 

 circuit made up of various electrolytes, one set giving rise to positive 

 actions and the other to negative actions. If a denotes the heat of 

 combination for unit weight of those of the first kind, and b for 

 those of the second, R the total resistance, and I the strength of the 

 current, we shall have 



or 



The product IR, which corresponds to the heat liberated in the 

 circuit owing to the resistances, being essentially positive, the current 

 could only exist provided that 



If this condition is not fulfilled, and all the electrolytes are at 

 first in the natural state, the current is established the moment the 

 circuit is closed. An incomplete decomposition polarizes the elec- 

 trodes, and the current ceases as soon as the sum of the electro- 

 motive forces of polarization attains the value ^ap ; the system 

 remains then in equilibrium. This is the case with a circuit formed 

 of a DanielFs cell (263) and a voltameter; the replacement of 

 copper by zinc in Dani ell's cells gives 24-2 thermal units, while the 

 decomposition of water requires 34*5. 



260. DEPOLARIZATION BY DIFFUSION. It may, however, happen 

 that an extremely feeble current is then observed. This current is 

 due to the following cause : the polarization of the electrodes of the 

 voltameter is gradually dissipated in consequence of the diffusion 

 of the gas ; it can be seen that this diffusion will be more or less 

 rapid according to the conditions of the experiment, but especially 

 according to the value of the polarization itself, and its deviation in 

 reference to the maximum polarization. The current observed in 

 these circumstances will be that necessary to re-establish the losses 

 due to diffusion, and to maintain the state of equilibrium which 



