MOTIONS OF ELECTRONS 175 



would pass through the air. This miniature lightning, 

 Galvani noticed, caused twitchings of the muscles of 

 the frogs' legs. This started him upon a series of ex- 

 periments as to the effect of electricity upon vital 

 actions, in the course of which he was rewarded by 

 another accidental discovery. Some frogs 7 legs, hung by 

 copper hooks from an iron railing, convulsed violently 

 when they swung into contact with the railing. Guided 

 by this he studied the effect further. He arrived, how- 

 ever, at a wrong explanation of it, assuming that at 

 the junction of the nerve and the muscle there was a 

 separation of electricities. 



It remained for Volta, a professor at Pavia, to show 

 about 1800 that the source of electrification was in the 

 dissimilar metals and was made available for continuous 

 effect if the two metals were separated by a liquid like 

 salt water. From this accidental start there was de- 

 veloped the science of electricity as we know it to-day. 

 Men have forgotten the original experiment, which 

 Galvani had in mind to do, but the by-product has had 

 far reaching effect. His name is preserved in science 

 in the word " galvanometer," meaning an instrument 

 for metering "galvanic" or " voltaic" currents. 



Volta's first form of electric battery was the "pile," 

 consisting of successive layers of copper, zinc, and wet 

 cloth in the order named. A later arrangement was a 

 series of glass vessels, each containing salt water and 

 plates of copper and zinc. These plates did not make 

 contact inside the vessels but were connected outside 

 them from copper to zinc as illustrated schematically 

 in Fig. 16. In each vessel the zinc plate was found to 

 be negatively and the copper plate positively electrified. 



