220 THE ELECTRIC TELEGRAPH. 



(a) in one finger of the right hand and one finger of the 

 left; or 



(>) in two fingers of the right ; or, lastly, 



(c) in two fingers of the left hand. 



To the remaining two effects of the galvanic current the 

 chemical and the magnetical we are indebted for the oppor- 

 tunities which we have of measuring and studying more 

 intimately its laws. 



To the chemical effects, besides the galvano-plastic art 

 and all the ramifications of industry which it has been the 

 means of introducing, we owe the decomposition of water 

 and the discovery of many of the metals. 



The decomposition of water, made by M. Sommering the 

 basis of his telegraph, has been found to be a just measure 

 of the current producing it, and has therefore been used as 

 such. The decomposition of salts is employed in the chemical 

 telegraphs of Gfintl, Bonelli, and others, and promises to come 

 ultimately into more extended use. 



The magnetic effects of the galvanic current are manifested 

 in the deflection of a magnetic needle suspended near a wire 

 in which a current is moving; in the magnetisation of a 

 soft iron, when a current circulates round it ; and in the 

 induction of currents in close circuits in the neighbourhood of 

 a current, at the moment of its appearance and disappearance. 



3. Galvanic Batteries. Cruikshank remodelled the pile 

 of Yolta, and gave it the form of a trough divided into several 

 compartments, each of which contained a pair of zinc-copper 

 plates immersed in dilute sulphuric acid, instead of the cloth 

 discs moistened with acidulated water, as in Yolta' s pile. 



Wollaston improved this form of battery, first, by entirely 

 surrounding each zinc plate by the copper plate, and secondly, 

 by making it a plunge battery, the electro-motors being 

 arranged at distances along a non-conducting bar, which, 

 when lowered in its frame, plunged each of the plates into its 

 proper vessel of acidulated water. These elements had a con- 

 siderable electro-motive force when first set up, but went down 

 very soon afterwards, through polarization of the copper 

 plates, by the hydrogen gas collecting on their surfaces. 



