HISTORY AND PROGRESS. 17 



will be perceived according as the conductor is being charged 

 with positive or negative electricity. The similarity of these 

 results with those obtained by Sulzer was an analogy 

 advanced by Yolta in support of his contact theory. 



Another theory, and that now very generally accepted, was 

 first suggested by Fabroni, and is known in contradistinc- 

 tion as the chemical theory. This theory regards chemical 

 decomposition as necessary to the development of the voltaic 

 current. 



A discussion of the arguments advanced in support of these 

 two theories would be out of place here. M tiller has devoted 

 an excellent chapter to the subject in one of his books.* The 

 German physicists for the most part hold out for the contact 

 theory, whilst the French and English generally accept the 

 chemical as the most rational and as that affording the most 

 satisfactory explanations of known phenomena. 



21. Volta, in 1800, wrote a letter from Como to Sir Joseph 

 Banks, President of the Royal Society, in London, stating 

 that he had found a means of augmenting, at pleasure, the 

 development of galvanic electricity. This he had accom- 

 plished by placing upon a plate of glass first a disc of copper, 

 then on this a disc of zinc, and over these a similar sized 

 disc of damp cloth ; and by continuing to pile up discs of 

 these materials, in the same order, copper, zinc, cloth, until 

 he had a sufficient number. He then connected wires to the 

 lower and upper plates. 



22. This apparatus is known as the Yoltaic pile.f Its pro- 

 perties are concisely stated as follows : 



1st. It communicates a charge of positive electricity to a 

 condenser in connection with the wire attached to the last zinc 

 disc, when the last copper disc is connected with the earth. 



2nd. It communicates a charge of negative electricity to 

 the condenser when the poles are reversed, that is to say, when 

 the zinc of the upper part is put into contact with the earth, 

 and the condenser with the copper disc at the bottom. These 



* " Fortschritte der Physik," vol. i. p. 225. 



f The more recent forms given to the pile, as at present employed in tele- 

 graphy, are explained in the second part. 



