GALVANIC BATTERIES. 



223. 



Professor Stewart : " The zinc dissolves in the dilute sulphuric acid, and 

 during this process hydrogen gas is given off. But this hydrogen does not 

 rise up in the shape of bubbles ; it finds its way into the porous vessel which 

 contains the strong nitric acid. It there decomposes the acid, taking some 

 oxygen to itself, so as to become water (hydrogen and oxygen forming 

 water), and thereby turning the nitric into nitrous acid, which shows its 

 presence by strong orange-coloured fumes." By this decomposition of the 

 nitric acid the polarization of the platinum (due to hydrogen) is avoided. 

 The porous cell, while keeping the liquids apart, does not interfere with the 

 chemical action. 



A great number of cells are used in the Grove Battery; perhaps 

 even a hundred may be employed. 



Smees Battery consists of a plate of platinized silver, S, with a bar of 

 wood to prevent contact with the zinc on each side, Z. These are immersed 



Fig. 225. Smee's Cell 



Fig. 226. Smee's Battery. 



in a glass jar, A, which contains dilute sulphuric acid. The current is 

 obtained by metallic communication with the binding-screws on the top. 

 This battery has much the same general arrangement as Wollaston's the 

 position of the plates being, however, reversed ; in the latter there are two 

 negative plates to one positive. In Smee's Battery there are two positive 

 (zinc) plates to one negative plate. 



It will now be understood how an electric current is produced ; the 

 electricity passing through the cells, etc., to wires, confers certain properties 

 upon the wires, and we can ascertain the effect of the current by means of a 

 Galvanometer, an instrument used to detect the strength and direction of elec- 

 tric currents. The current will evolve heat and light ; it will excite muscular 

 action, and will decompose substances into their constituent elements. The 

 deflection of the magnetic needle by the electric current is considered the 

 best evidence of its power ; it is on this that the Galvanometer is based. 



We can perform a few simple experiments with the current. Suppose, 

 for instance, that a piece of fine wire be fixed between the pole wires of the 

 battery ; it will be heated " white hot." Or if two carbon points be 

 approached in a glass of water, as in the illustration (fig. 227), they will emit a 

 brilliant light in the fluid from the voltaic arc which has given us the electric 

 light. The current is the passage of electricity along the wire, and continues 



