244 ENERGY OF CURRENTS. 



and then goes on progressively diminishing until the thickness of 

 the layer becomes so great that the fresh bubbles no longer expe- 

 rience an action on the part of the plate, and can then escape freely. 



The work of the decomposition of water only attains its 

 normal value from this period. Hitherto the normal value has 

 been diminished by the work of condensation in question; experi- 

 ment shows that at the outset the value of this difference may 

 be very small. 



The modification which the surface of the plates thus undergoes is 

 the cause of the phenomenon known as polarization of the electrodes, 

 and which manifests itself by the development of an electromotive 

 force opposed to that which produces the current. We can thus 

 understand how it is that this polarization is not instantaneous, that 

 it may increase continuously from zero to a maximum limit; and, 

 finally, how the quantity of electricity required to produce a given 

 state of polarization depends on the condition and dimensions of 

 the plates. This quantity is often called the capacity of polarization 

 relative to the given system. 



By taking electrodes of very unequal surfaces and passing a given 

 quantity of electricity at a given potential through the voltameter, 

 we can produce polarization of either electrode at will; recent 

 experiments by M. Blondlot show that the phenomenon follows the 

 same law whatever be the direction of the current, and that for a 

 given electrode and given electrolyte the capacity does not depend 

 on the direction of the polarization. 



253. SECONDARY CURRENTS. When once polarization is set 

 up, if the original current is opened and the two electrodes are 

 themselves joined by a wire, the electromotive force of polarization 

 H, produces a current in a direction opposite to that of the original 

 current, but the current rapidly diminishes and finally disappears 

 more or less completely ; this current is called the secondary current. 



It is easy to account for this phenomenon ; when the two elec- 

 trodes are connected by a conductor, the layer of gas gradually 

 disappears, reforming water ; the electromotive force diminishes and 

 disappears with it; and lastly it is clear that the total quantity of 

 electricity set in motion while the secondary current lasts, must be 

 equal to that expended in effecting the polarization of the electrodes. 



It is manifest that the current would remain constant provided 

 the electromotive force H could be kept constant; it would be 

 sufficient for this if the layer of gas necessary for complete polariza- 

 tion were maintained at the surface of the electrode. This is 

 precisely what takes place in Grove's gas battery. 



