382 ELEMENTARY LESSONS ON [CHAP. XL 



while at the other pole copper is deposited by the 

 current out of a solution of copper sulphate, thereby 

 setting up an opposing (or -) electromotive-force. 

 That due to zinc is shown above to be + 2-396 

 volts, that to deposited copper to be i'223. 

 Hence the net electromotive - force of the cell is 

 (neglecting the slight electromotive-force where the 

 two solutions touch) 2^394 - 1*223 = 1*171 volts. 

 This is nearly what is found (Art. 178) in practice 

 to be the case. It is less than will suffice to 

 electrolyse water, though two Daniell's cells in 

 series electrolyse water easily. 



415. Secondary Batteries : Storage of Electric 

 Currents. A voltameter, or series of voltameters, whose 

 electrodes are thus charged respectively with hydrogen 

 and oxygen, will serve as secondary batteries, in which 

 the energy of a current may be stored up (as chemical 

 work) and again given out. Ritter, who first constructed 

 a secondary battery, used electrodes of platinum. Gaston 

 Plant^ employs two pieces of sheet lead rolled up (with- 

 out actual contact) as electrodes, dipping into dilute 

 sulphuric acid, as in Fig. 154; the lead becoming with 

 use coated with a semi-porous film of brown dioxide of 

 lead presenting a large amount of surface and holding 

 the gases well. When such a battery, or accumulator of 

 currents, is charged by connecting it with a dynamo- 

 electric machine or other powerful generator of currents, 

 the anode plate becomes peroxidised, while the kathode 

 plate is deoxidised by the hydrogen that is liberated. 

 The plates may remain for many days in this condition, 

 and will furnish a current until the two lead surfaces are 

 reduced to identical chemical state. 1 The electromotive- 

 force of such cells may even attain from 2-38 to 272 volts. 

 Plant^ has ingeniously arranged batteries of such cells 

 so that they can be charged in parallel arc and discharged 



1 Faure has recently modified the Plante accumulator by giving the lead 

 plates a preliminary coating of red lead (or minium). Cells thus prepared 

 sooner acquire the effective spongy brown surface of dioxide of lead. 



