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XXYI. On the Contact Theory {Attraction of Unlike Metals). 

 Part III. By Signor Quirino Majorana*. 



ACCORDING to the hypothesis of Volta unlike metals in 

 metallic communication are at different electric poten- 

 tials ; they should therefore, when suitably arranged, attract 

 each other. This attraction has not previously been exhibited 

 experimentally; indeed, Lord Kelvin f, in a recent memoir, 

 expresses the opinion that it would be very difficult, if not 

 quite impossible, to demonstrate by experiment the attraction 

 between two disks of different metals. As a matter of fact, 

 in the case of two plane parallel disks in air no attraction has 

 been hitherto observed ; but by the use of special devices the 

 attraction between two pieces of metal can be clearly 

 demonstrated. 



The object of the present note is to describe experiments 

 illustrating the phenomenon alluded to. 



Consider, in the first place, two unlike metallic bodies : 

 after having been connected metallically or after having been 

 discharged to earth, they are at different potentials. The 

 electric layers surrounding the two metals exert a mutual 

 force which is in general an attraction. The gravitational 

 attraction, which acts at the same time between the metals, 

 usually differs from the electric force. In the experiments 

 described in the present paper the attractions obtained may 

 be considered as due to the electric force, the gravitational 

 attraction between the small masses being of much smaller 

 magnitude. 



The experimental arrangement which has given the best 

 results is the following : — A quartz fibre of diameter 0"01 

 millim., or even less, and about 10 centim. long, is silvered 

 over the whole of its surface. Its upper end, after having 

 been plated with copper for a short distance, is soldered to an 

 insulated wire of stiff copper. 



The lower end, greatly enlarged, is represented by Q in 



* From the Rendiconti delict R. Accademia del Lincei, viii. [l]pp. 302- 

 309 (1899): translated by Dr. J. L. Howard, University College, Liver- 

 pool, from a separate impression communicated by Lord Kelvir. 



Since the publication of the two previous notes on the Contact Theory, 

 the author has learned that Prof. Rig'hi, in a memoir written some time 

 ago {SuW aztone del eoibenti nelle esperienze di elettricita di contutto, Ace. 

 dei XL. [3] p. 15) described the production of charges of contrary sign to 

 those of the Volta experiment, by causing two unlike metals to approach 

 each other. Prof, liighi's discovery was made before the experiments of 

 Exner alluded to in the first note. 



t Phil. Mag. July 1898, p. 104. 



