i66 GREAT MEN OF SCIENCE 



conductors of the second class, and he laid stress on the con- 

 tinual circulation of electricity through the three conductors, 

 which together form a circuit as long as they are in contact. 

 The seat of the driving force ('electro-motive force' as we say 

 to-day) he regards quite generally as lying in all three places 

 of contact. 



Volta now proceeds to eliminate organisms entirely from 

 his experiments. The nerve or the tongue became finally for 

 him only means of testing for electrification; he attempts now 

 to replace them entirely by the electroscope with condenser. 

 The undertaking was very difficult to carry out unexcep- 

 tionably, on account of the smallness of the forces at work; but 

 in August 1796 Volta was able to announce that he had 

 finally succeeded, beyond his expectations, in making the 

 electrification resulting from the contact of two metals only, 

 'actually perceptible.' This was the fundamental experi- 

 ment of Volta, still famous to-day. He succeeded only by 

 using in connection with the electrometers of that period 

 repeated multiplication by means of the condenser, a 

 method in which only a very skilful and careful experi- 

 menter can avoid deceiving himself. 



Thenceforward the way was somewhat easier. Volta 

 proceeded quantitatively, although only approximately, 

 which however, is generally quite sufficient in the first 

 pioneer work, when the experiments are pure in their re- 

 sults. By investigating pairs of conductors of the first class, 

 he was able to arrange them all in a series, since called the 

 'Voltaic series,' in such a way that the electromotive force or 

 electrical tension produced by any pair of conductors to- 

 gether, is proportional to their distance apart in the series. 

 At the top of the series stands zinc; at the bottom, carbon; 

 about in the middle, copper; the higher member is always 

 positive towards the lower. This allows us to see that con- 

 ductors of the first class alone do not permit a continual cur- 

 rent of electricity to be produced, since the tensions at the 



