158 The Evolution of Electric and Magnetic Physics. 



"When their bodies were brought into the vicinity of the 

 machine it was noticed that their legs twitched. The oper- 

 ator commenced to reflect upon this phenomenon, and the 

 result of his reflection and further investigation was the 

 discovery of galvanism, as it was called from his name. The 

 subject afterward received much attention. The results of 

 that discovery were developed and followed up with greater 

 success by Volta, who published in 1800 his first account of 

 the voltaic pile, the lineal ancestor of all the different bat- 

 teries with which we are to-day familiar, from which they 

 have been differentiated by a natural process of evolution. 

 Volta made use of Galvani's experiments, but he arrived in- 

 dependently at new and important results, and so is entitled 

 to equal credit with Galvani. Galvani had supposed that 

 the electricity came from the frog. Volta showed that it 

 resulted from the contact of dissimilar substances. 



Much dispute arose at the time as to the source of the 

 electrical activity in the generator, and up to this date the 

 question is yet open ; but it appears safe to conclude that 

 electrical activity is capable of being developed whenever 

 two dissimilar substances, or even dissimilar parts of the 

 same substance, are brought into contact. If the two sub- 

 stances are non-conductors, as, for example, glass and silk, 

 the electrical force indicates a high tension or pressure, 

 especially when the substances are rubbed together. It 

 would burst through the air in sparks, across a considerable 

 space. This was the condition of the electricity generated 

 before the time of Galvani. On the other hand, Avhen the 

 substances in contact are conductors, such as metals, no ac- 

 cumulative effect is obtained by the mutual friction of their 

 surfaces, and the electricity produced, while it may be abun- 

 dant in quantity, is of low pressure, and can not burst 

 through the air in sparks, or evince powerful attraction for 

 surrounding objects. The electricity in each case is sup- 

 posed to be identical in nature, but to differ in behavior, 

 through differences in the pressure under which it acts. 



Many years were passed in establishing this relationship 

 between statical or frictional and dynamic or galvanic elec- 

 tricity, but now galvanic effects can be obtained from fric- 

 tionally produced electricity, and all the phenomena peculiar 

 to so-called static electricity can be produced by increasing 

 sufficiently the number of galvanic cells, and thus accumu- 

 lating their pressure. This property of pressure, or its elec- 

 trical analogue, is quantitatively expressed in terms of a unit 



