Makch 24, 1916] 



SCIENCE 



409 



ON THE NATURAL CHARGES OF 

 METALS 1 



In 1789 Bennett discovered that when two 

 similar, insulated brass plates are placed very 

 close together and parallel to each other and 

 are simultaneously touched with pieces of 

 different metal held in the hands they become 

 charged relative to each other, and oppositely 

 charged relative to the earth. Bennett gave 

 the results of touching his plates with six 

 different pairs of metals, and thus laid the 

 foundation for what later came to be called 

 the Volta contact series of metals. Bennett 

 concluded as the result of his experiments that 

 different substances have " a greater or less 

 affinity with the electrical fluid," and he pub- 

 lished a series of " Experiments on the Ad- 

 hesive Electricity of Metals and Other Con- 

 ducting Substances." 



Bennett also tried the effect of touching one 

 brass plate with a single metal while the other 

 plate was parallel and very close to it but was 

 joined to earth, and he found that his brass 

 plate would take a positive charge when 

 touched with lead ore, gold, silver, copper, 

 brass, regulus of antimony, bismuth, tutenag, 

 mercury and various kinds of wood and stone ; 

 but that it would take a negative charge from 

 zinc and tin. 



Six years later (in 1795) Cavallo published 

 the results of a series of experiments on con- 

 tact electrification. Oavallo placed a tin plate 

 upon insulating supports and dropped a piece 

 of metal upon it from the hand or from tongs 

 or a spoon. He then tilted the tin plate and 

 allowed the metal to slide off it, after which 

 it was picked up and dropped onto the plate 

 again. By sufficient repetition of this proc- 

 ess, the plate became so highly electrified that 

 the nature of its charge could be determined. 

 Cavallo tried the effect of dropping his pieces 

 of metal from a spoon or tongs of another 

 metal, and made a large number of experi- 

 naents upon the effect of heating or cooling 

 the pieces of metal before they were dropped 



1 Read before a joint meeting of Section B of 

 the American Association for the Advancement of 

 Science and the American Physical Society, at 

 Berkeley, California, August 5, 1915. 



upon the tin plate. At the end of his experi- 

 ments, he said, among his other conclusions : 



I am inclined to suspect that different bodies 

 have different capacities for holding the electric 

 fluid, as they have for holding the elementary heat. 



In the meantime, Volta had discovered the 

 existence of an electric current in a circuit 

 made of two metals and a moist conductor. 

 He first thought the source of the current to 

 be in the surfaces of contact of the metals with 

 the moist conductor, but later concluded that 

 the current was not only originated, but was 

 sustained, by the mutual contact of two metals 

 of a different kind. In support of this con- 

 clusion, he published a series of experiments 

 on contact electrification which were a virtual 

 repetition of Bennett's experiments which had 

 been published eight years before, but for 

 which Volta gave Bennett no credit. 



Meanwhile, in 1792, Fabroni had published 

 his celebrated paper entitled " Upon the Chem- 

 ical Working of the Different Metals upon 

 Each Other at Ordinary Air Temperatures, 

 and Upon the Explanation of Certain Gal- 

 vanic Phenomena." In this paper Eabroni 

 showed that the surface cohesion of different 

 metals is changed merely by their mutual con- 

 tact, so that metals which before contact were 

 not attacked by the oxygen of the air or of 

 water are readily oxidized when in contact 

 with another less oxidizable metal. When 

 Volta's discovery of the current was an- 

 nounced, Fabroni naturally concluded that the 

 chemical action which took place at the sur- 

 face of contact of at least one of the metals 

 and the moistened membrane was the cause 

 of the electrical current. 



As a result of the controversy which fol- 

 lowed regarding the source of the electro- 

 motive force in the voltaic current, a similar 

 controversy arose over an entirely different 

 question, viz., as to whether the transference 

 of electricity from one metal to another as 

 observed by Bennett and Cavallo was a pri- 

 mary phenomenon of metallic contact, or 

 whether it was due to a preceding action of 

 oxygen or some other element upon one or 

 both of the metals. Ostwald, speaking of the 

 theory of direct electrification by contact says : 



