140 ELEMENTARY LESSONS ON [CHAP. in. 



actions may be represented as taking place in two stages. 

 Zn + H 2 SO 4 Zn SO 4 + H 2 



Zinc and Sulphuric Acid produce Sulphate of Zinc and Hydrogen. 



And then 



H 2 + Cu SO 4 = H 2 SO 4 + Cu. 



Hydrogen and Sulphate of Copper produce Sulphuric Acid and Copper. 



The hydrogen is, as it were, translated electro-chemi- 

 cally into copper during the round of changes, and so 

 while the zinc dissolves away the copper grows, the dilute 

 sulphuric acid gradually changing into sulphate of 

 zinc, and the sulphate of copper into sulphuric acid. 

 There is no polarisation, and the battery, though not 

 powerful (owing to its internal resistance), is quite con- 

 stant, 1 and hence affords a valuable standard of comparison 

 by which to measure the electromotive - force of other 

 batteries. When the dilute acid used consists of one part 

 (by weight) of acid to twelve parts of water, the E.M.F. 

 of one element is 1-079 volts. Owing to its constancy 

 this battery, made in a flat form, many cells of which 

 can be readily set up side by side in a wooden trough 

 divided into partitions (see Fig. 77), is much used in tele- 

 graphy. 



171. Grove's Battery. Sir Wm. Grove devised a 

 form of battery having both greater E.M.F. and smaller 

 internal resistance than Daniell's Cell. In Grove's 

 element there is an outer cell of glazed ware or of 

 ebonite, containing the amalgamated zinc plate and 

 dilute sulphuric acid. In the inner porous cell a piece 

 of platinum foil serves as the negative pole, and it dips 

 into the strongest nitric acid. There is no polarisation 

 in this cell, for the hydrogen liberated by the solution of 

 the zinc in dilute sulphuric acid, in passing through the 



1 That is to say, sufficiently so for ordinary purposes. Mr. Latimer Clark 

 says that its E.M.F. may vary between 1*079 an d 0*978 volts, according to its 

 condition. For very exact testing he suggests a standard cell described in 

 Art. 177. 



