206 OUR PHYSICAL WORLD 



that the muscles of the leg of a dead frog will twitch if the nerve 

 in them is excited by frictional electricity. Having prepared 

 several frogs' legs for further experiments, he hung each by a 

 copper wire to an iron railing of the balcony outside his window. 

 As they blew about in the wind, he noted with surprise that when- 

 ever one of the legs was thrown against the iron it was convulsed 

 with a contraction. He thought the electricity that caused 

 this must be generated by the animal and resided in its tissues. 



When Alessandro Volta, a professor of natural philosophy 

 at the University of Pavia in Italy, heard of this he repeated 

 the experiment, but suspected that the electricity was coming 

 from the copper and iron, bathed with moisture from the 

 tissues. So he placed several cups in a small circle on the 



table, and filled them partly 

 full of water. In each cup he 

 stood a strip of zinc and, op- 

 posite it, one of copper so that 

 the upstanding end of one 

 copper strip leaned against the 



zinc strip in the next cup. The 

 FIG. 8 3 .-Volta's crown of cups cup 



not touch each other (Fig. 83) . If, now, one copper strip was sep- 

 arated, by ever so little, from the zinc strip against which it leaned, 

 a tiny spark appeared at the gap, showing that a current of electri- 

 city was being developed. He tried adding various substances 

 to the water in the cups to see if the strength of the current 

 might be increased. He found that any acid would do this 

 very efficiently. Then he improved his apparatus. He piled 

 up alternate plates of zinc and copper, separating them by 

 flannel pads wet with acid, but connecting each plate with those 

 on each side of it by short wires. One end of a wire, the 

 other end of which was attached to the lowest zinc plate, was 

 brought close to the free end of another similarly attached to the 

 top copper plate. A much brighter spark showed a more power- 

 ful current. This device is still known as a voltaic pile, and we 



