CHAP. I.] ELECTRICITY AND MAGNETISM. 69 



72. Volta found, moreover, that the differences of 

 electric potential between the different pairs of metals 

 were not all equal. Thus, while zinc and lead were 

 respectively + and to a slight degree, he found zinc 

 and silver to be respectively + and to a much greater 

 degree. He was able to arrange the metals in a series 

 such that each one enumerated became positively elec- 

 trified when placed in contact with one below it in the 

 series. Those in italics are added from observations 

 made since Volta's time 



CONTACT SERIES OF METALS (IN AIR). 



+ Sodium. 



Magnesium. 



Zinc. 



Lead. 



Tin. 



Iron. 



Copper. 



Silver. 



Gold. 



Platinum. 

 Graphite (Carbon). 



Though Volta gave rough approximations, the actual 

 numerical values of the differences of potential for 

 different pairs of metals have only lately been measured 

 by Ayrton and Perry, a few of whose results are tabu- 

 lated here 



Difference of Potential 

 (in volts). 



Zinc 



Lead 



Tin 



Iron ' ' -3*3 



