250 ENERGY OF CURRENTS. 



but the water is soon decomposed, oxygen goes against the current 

 and oxidizes the zinc plate, while hydrogen goes along with the 

 current and polarizes the copper plate ; from this follows an inverse 

 electromotive force. When the stationary condition is established, 

 the electromotive force of polarization is E', and the strength V 

 satisfies the ratio 



(E-E')r = r 2 R, 



or 



R 



If the couple is allowed to rest, the polarization disappears slowly, 

 owing to diffusion. When it is again closed after the lapse of some 

 time, the current at first reappears with its original strength I (if the 

 influence of the layer of zinc oxide may be neglected), to regain the 

 intensity I' after a lapse of time which is usually very short, but which 

 may be very long if the surfaces of the electrodes are very large and 

 the resistance of the circuit is considerable. As long as the couple 

 is open, the difference of potential of the extremities is equal to E. 



In a closed circuit the available electromotive force is E E'. In 

 each couple, oxide of zinc and hydrogen are produced at the expense 

 of the zinc and of the water. As we may assume that the oxygen has 

 passed through the gaseous state in going from the water to the zinc, 

 it will be seen that the disposable energy of the couple, corresponds 

 to the excess of the heat of formation of the oxide of zinc over that 

 of the formation of water for the same weight of oxygen. If the 

 water is acidulated, the difference corresponds to the substitution 

 of zinc for hydrogen in sulphuric acid: this difference is about 177 

 thermal units. 



The layer of hydrogen which covers the copper has also the 

 effect of greatly increasing the resistance of the couple, which is a 

 fresh cause for the enfeeblement of the current. 



262. UNPOLARIZABLE CELLS. Mechanical means, such as the 

 agitation of the liquid, or rubbing the copper plate with a foreign 

 body, greatly diminish the resistance, and even the polarization, by 

 getting rid of the greater part of the gas ; the layer of gas may be 

 completely removed by chemical action, and thus non-polarizable 

 couples or cells be obtained. 



A liquid which merely dissolved the hydrogen without calorific 

 action, would increase the electromotive force by the whole amount 

 of the work which the gas performs in filling a given volume at the 

 external pressure ; but if the hydrogen enters into a new chemical 



