694 INDUSTRIAL APPLICATIONS. 



the same result more rapidly by first immersing the lead plates 

 for a day or two in nitric acid diluted with its own bulk of water. 

 With a small loss of weight, the metal experiences a kind of 

 scraping or roughening, which doubtless increases its porosity and 

 facilitates the ultimate penetration of the oxidation produced by 

 the primary current. 



In any case when a formed accumulator is polarized to excess, 

 the electrode which becomes positive in the discharge supports a 

 certain quantity of binoxide of lead PbO 2 , and the negative elec- 

 trode an equivalent quantity of metallic lead in the porous condition. 

 The couple being closed, the positive electrode is reduced, the other 

 becomes oxidised ; the electromotive force of the secondary couple 

 retains a constant value for the greater part of the discharge, and 

 then diminishes slowly until the depolarization is complete. 



In practice it is desirable to use only the first part of the opera- 

 tion, that which corresponds to a constant electromotive force ; the 

 accumulator is then charged anew^ either by a battery or by any 

 other electromotor. It is advantageous to group the accumulators 

 abreast during the charge, so that only a small electromotive force 

 has to be overcome, and to arrange them in series for the discharge. 



1250. The investigation of the reactions which take place in the 

 discharge of an accumulator is somewhat difficult. According to 

 the thermochemical researches of M. Tscheltzow,* sulphate of lead 

 simply is found on the two electrodes, according to the equation 



Pb + PbO 2 + 2SO 3 + Aq = 2PbO.SO 3 + Aq. 



The sum of the quantities of heat liberated in this reaction 

 corresponds in fact to i -93 volts ; and sensibly represents the electro- 

 motive force of the accumulator. 



As the equivalent of lead is 103-5, the weight of lead in the 

 binoxide which corresponds to the passage of a coulomb (1242) is 

 1-0712 mgr., so that a kilogramme of lead, half .of which comes into 

 play on each electrode, may furnish 



SOO.OOO 



= 467,000 coulombs. 

 1-0712 



TSCHELTZOW. Comptes rendus, Vol. c., p. 1458. 1885. 



