PHYSICS OF NINETEENTH CENT. ELECTRICITY. 553 



in the number, variety, and importance of its ultimate scientific con- 

 sequences than this one of Volta's. VOLTA (1745 1826) had been the 

 Professor of Natural Philosophy in the University of Pavia, and had 

 retired from the duties of that position when, at Como, he made the 

 great discovery which was first announced to the scientific world in a 

 letter to the Royal Society of London. This letter, dated the 2oth of 

 March, 1800, was read before the Society on the 26th June, 1800. 

 He says that he has made an apparatus which, in some of its effects, 

 is like a Leyden jar feebly charged, but constantly recovers its charge, 

 and thus forms a perpetual source of electricity, and is, moreover, 

 different from other electrical apparatus in being made entirely of 

 conducting substances. It is formed of a number of discs of copper, 

 zinc, and paper or leather steeped in salt water. 

 He compares it to the electric organs of the torpedo 

 and similar animals. The discs may be about an 

 inch in diameter, but their shape and size are im- 

 material. As silver gives better results than copper, 

 he sometimes uses silver coins, and thus constructs 

 his apparatus : he places a silver coin on a hori- 

 zontal support, and above the coin a disc of zinc. 

 Upon the zinc he lays a disc of moistened leather 

 or paper, above that another silver coin, then again 

 a disc of zinc, followed by one of paper, and so on 

 until a column has been formed containing several 

 dozens of each kind of discs, always arranged in the 

 same order. Volta explains how the apparatus gave 

 signs of electricity and a spark might be obtained, 

 and at the moment when the fingers were made to 

 touch simultaneously the lowest and highest of the 

 metallic discs, a slight electrical shock was felt. 



Such was the apparatus which became famous 

 under the name of the Voltaic Pile (see Fig. 264). 



Volta himself seems to have had no idea of the 

 many capabilities of his apparatus. He is content 

 with finding only those we have already mentioned, 

 and he passes on to show that his apparatus is an FlG 26 _VOLTAIC 

 " experintcntum cruris" in favour of the contact PILE. 



theory. The action by which the electric fluid is 

 put into operation does not, he contends, take place where the moist 

 substance touches the metal, though perhaps some insignificant effect 

 might even there be also produced ; but its chief seat is at the contact 

 of the two dissimilar metals, e.g., of the silver and the zinc. The 

 layers of moisture are, he says, only the medium of connecting the 

 different metallic couples by which the electric fluid is driven in one 

 direction throughout. He found, indeed, that when the discs were 

 moistened with a saline or acid solution, the effects were far more 



