ELECTRICITY. 75 



phuric acid, many other materials may be used to bring about the 



conditions described in the preceding paragraph ; indeed, electricity 



is generated whenever two solids (plates or cylinders are generally 



used), conductors themselves and connected by a conductor, are 



placed in a liquid that has the power to act chemically on one of the 



solids. Any such arrangement is termed 



a galvanic or voltaic cell. Here chemical FIG. 29. 



action causes the generation of electricity, 



resulting in the zinc-platinum cell in the 



splitting up of sulphuric acid, H 2 SO 4 , 



the hydrogen, H 2 , escaping from the 



platinum plate, while the group SO 4 



combines with the zinc, forming zinc 



sulphate, ZnSO 4 , which dissolves in the 



water. 



Many combinations are employed in 

 the different cells for generating elec- 

 tricity. Fig. 29 represents the Daniell Danieii's ceil. 

 cell. It consists of the glass jar S, con- 

 taining a saturated solution of cupric sulphate, in which stands the 

 copper cylinder C. Inside of this cylinder fits a porous cell, A, con- 

 taining sulphuric acid, and into this dips the zinc plate. Z. 



Chemically pure zinc is scarcely acted on by diluted sulphuric acid ; ordi- 

 nary zinc contains metallic impurities, and as these are in contact with zinc, 

 they set up a galvanic action, and thus bring about the chemical changes above 

 described. 



What is believed to take place in any galvanic cell is that the 

 electrolytic fluid i. e., the active agent in the liquid state is split 

 up into two component parts, called ions, and that these ions, charged 

 with positive and negative electricity, respectively, unload these 

 charges on the two plates, while an equalizing effect is brought about 

 through the connecting wire. In other words, there is a constant 

 disturbance of the electrical equilibrium through chemical action, 

 and a tendency to re-establish the equilibrium, which tendency pro- 

 duces the current. 



One of the plates (platinum, in the case cited above) is known 

 as the positive electrode or anode, while the other one (zinc) is the 

 negative electrode or cathode of the cell. The same terms are used 

 also to designate the terminals of the wires leading from the two 

 plates, which terminals are called also + and poles. 



