Galvanism, from Galvani to Ohm. 73 



oate electroscope, which indicated by the divergence of its straws 

 that the disks were now electrified the zinc had, in fact, acquired 

 a positive and the copper a negative electric charge.* Thus the 

 mere contact of two different metals, such as those employed in / 

 the pile, was shown to be sufficient for the production of effects ' 

 undoubtedly electrical in character. 



On the basis of this result Volta in the same year (1800) 

 put forward a definite theory of the action of the pile. Suppose 

 first that a disk of zinc is laid on a disk of copper, which in turn 

 rests on an insulating support. The experiment just described 

 shows that the electric fluid will be driven from the copper to 

 the zinc. We may then, according to Volta, represent the state 

 or " tension " of the copper by the number - J, and that of the 

 zinc by the number + J, the difference being arbitrarily taken as 

 unity, and the sum being (on account of the insulation) zero. It 

 will be seen that Volta's idea of " tension " was a mingling of 

 two ideas, which in modern electric theory are clearly distin- 

 guished from each other namely, electric charge and electric 

 potential. 



Now let a disk of moistened pasteboard be laid on the zinc, 

 and a disk of copper on this again. Since the uppermost 

 copper is not in contact with the zinc, the contact-action does 

 not take place between them ; but since the moist pasteboard is 

 a conductor, the copper will receive a charge from the zinc. 

 Thus the states will now be represented by - f for the lower 

 copper, + J for the zinc, and + \ for the upper copper, giving a 

 zero sum as before. 



If, now, another zinc disk is placed on the top, the states 

 will be represented by - 1 for the lower copper, for the lower 

 zinc and upper copper, and + 1 for the upper zinc. 



In this way it is evident that the difference between the 

 numbers indicating the tensions of the uppermost and lowest 



* Abraham Bennet (b. 1750, d. 1799) had previously shown (Xew Experiments 

 in Electricity, 1789, pp. 86-102) that many bodies, when separated after contact, f 

 are oppositely electrified ; he conceived that different bodies have different attrac- 

 tions or capacities for electricity. 



