CHAP, in.] ELECTRICITY AND MAGNETISM. 135 



opposing electromotive-force; for hydrogen is almost as 

 oxidisable a substance as zinc, especially when freshly 

 deposited (or in a " nascent " state), and is electropositive, 

 standing high in the series on p. 69. Hence the 

 hydrogen itself produces a difference of potential, which 

 would tend to start a current in the opposite direction to 

 the true zinc-to-copper current. 



It is therefore a very important matter to abolish this 

 polarisation, otherwise the currents furnished by batteries 

 would not be constant. 



165. Remedies against Internal Polarisation. 

 Various remedies have been practised to reduce or 

 prevent the polarisation of cells. These may be classed 

 as mechanical, chemical, and electro-chemical. 



1. Mechanical Means. If the hydrogen bubbles be 

 simply brushed away from the surface of the positive 

 pole, the resistance they caused will be diminished. If 

 air be blown into the acid solution through a tube, or if 

 the liquid be agitated or kept in constant circulation by 

 siphons, the resistance is also diminished. If the surface 

 be rough or covered with points, the bubbles collect more 

 freely at the points and are quickly carried up to the 

 surface, and so got rid of. This remedy was applied in 

 Smee's Cell, which consisted of a zinc and a platinised 

 silver plate dipping into dilute sulphuric acid ; the silver 

 plate, having its -surface thus covered with a rough coat- 

 ing of finely divided platinum, gave up the hydrogen 

 bubbles freely ; nevertheless, in a battery of Smee Cells 

 the current falls off greatly after a few minutes. 



2. Chemical Means. If a highly-oxidising substance 

 be added to the acid it will destroy the hydrogen bubbles 

 whilst they are still in the nascent state, and thus will 

 prevent both the increased internal resistance and the 

 opposing electromotive - force. Such substances are 

 bichromate of potash, nitric acid, and bleaching powder 

 (so-called chloride of lime). These substances, however, 

 would attack the copper in a zinc-copper cell. Hence 



