CHAP, in.] ELECTRICITY AND MAGNETISM. 123 



Continuous currents of electricity, such as we have 

 described, are usually produced by voltaic cells, or 

 batteries of such cells, though there are other sources of 

 currents hereafter to be mentioned. 



148. Discoveries of G-alvani and of Volt a. 

 The discovery of electric currents originated with Galvani, 

 a physician of Bologna, who, about the year 1786, made 

 a series of curious and important observations upon the 

 convulsive motions produced by the " return-shock " (Art. 

 26) and other electric discharges upon a frog's leg. He 

 was led by this to the discovery that it was not necessary 

 to use an electric machine to produce these effects, but 

 that a similar convulsive kick was produced in the frog's 

 leg when two dissimilar metals, iron and copper, for 

 example, were placed in contact with a nerve and a 

 muscle respectively, and then brought into contact with 

 each other. Galvani imagined this action to be due to 

 electricity generated by the frog's leg itself. It was, 

 however, proved by Volta, Professor in the University 

 of Pavia, that the electricity arose not from the muscle 

 or nerve, but from the contact of the dissimilar metals. 

 When two metals are placed in contact with one another, 

 one becomes positive and the other negative, as we have 

 seen near the end of Lesson VII., on p. 67, though the 

 charges are very feeble. Vcdta, however, proved their 

 reality by two different methods. 



149. Contact Electricity : Proof by the Con- 

 densing Electroscope. The first method of proof 

 devised by Volta involved the use of the Condensing 

 Electroscope, alluded to in Art. 71. It can be used in 

 the following way to show the production of electrifi- 

 cation. A small bar made of two dissimilar metals, zinc 

 and copper soldered together, is held in the hand, and 

 one end is touched against the lower plate, the upper 

 plate being at the same time joined to " earth " or 

 touched with the hand (Fig. 68). During the con- 

 tact electrical separation has taken place at the point 



