22 THE ELECTRIC TELEGRAPH. 



casting tie zinc on to a continuation of the copper plate. 

 This saves trouble, and avoids the danger attending the 

 coupling of the elements by metallic binding screws, from 

 oxidation of the ends, or loosely made connections. 



When the current passes the zinc is dissolved, and the 

 copper receives an equivalent increase in weight. In the 

 chamber containing the zinc and acidulated water, the oxygen 

 of each atom of water decomposed unites with an atom of 

 zinc, forming an atom of oxide of zinc, which in its turn 

 combines with an atom of sulphuric acid, forming sulphate 

 of zinc, which is dissolved in the water. The atom of hydrogen 

 released is transferred, by means of decompositions and re- 

 compositions, towards the copper cylinder. In the interior 

 of the porous pot an equivalent atom of sulphate of copper 

 is decomposed into one atom of copper, one of oxygen, and 

 one of sulphuric acid. The atom of copper is deposited upon 

 the plate by the current ; the atom of oxygen, moving to- 

 wards the zinc plate, meets the atom of hydrogen travelling 

 from the other compartment of the element, and combines 

 with it, forming together an atom of water ; while the atom 

 of sulphuric acid goes to the zinc compartment to renew the 

 supply there for the formation of sulphate of zinc, as that 

 metal is dissolved. 



When the circuit of such a battery is kept closed for a 

 length of time without addition to its constituents, the 

 sulphate of copper in the porous pot becomes all decom- 

 posed, and the water in the concentric chamber saturated 

 with sulphate of zinc. In such a state it is evident that the 

 advantages of the system are lost ; the hydrogen liberated 

 from the water, not meeting with the oxygen liberated 

 from the copper salt, polarises the copper plate, and lessens 

 the electro-motive force of the element. 



The periodical addition of crystals of sulphate of copper, 

 however, keeps up the saturation of the water in the inner 

 vessel, and, consequently, the constancy of the element. 

 But the water in the outer vessel should never be allowed to 

 get too saturated. 



A great inconvenience is always found in these elements 

 arising from the deposit of metallic copper at the bottoms 



