246 Big. Quirino Majorana on the Contact Theory. 



of facts which disprove the contact theory. In reality, many 

 of the statements of Exner are based upon experiments which 

 support the contact theory, although this was not recognized 

 by their author. The theory of Exner is founded upon the 

 single assumption that metals, when immersed in air, are 

 coated with exceedingly thin films of oxide. These films are 

 electrified, by the very fact of their formation, and would keep 

 their charges permanently because they are insulators. All 

 the phenomena observed and usually explained by the contact 

 theory are simply effects due to the electrostatic induction of 

 these films. 



This assertion has been made the subject of much criticism ; 

 nevertheless we are indebted to Exner for experiments which, 

 rightly interpreted, not only support the contact theory but 

 show clearly that the earth must be considered as one of the 

 bodies in Volta's series*. 



In consequence of the general hostility with which the 

 views of Exner have been received, several interesting experi- 

 ments described by him have not been accorded due con- 

 sideration. One of these, differently carried out, will be 

 found in the following pages. 



The object of this note, and of others to be presented later, 

 is to indicate some experiments which illustrate the theory of 

 contact, and which are mainly founded upon the extension of 

 Volta's principle previously described. 



Consider two disks, one of copper and the other of zinc. 

 Let them be placed in connexion with the earth and then 

 insulated. They assume, so far as can be seen, a definite 

 difference of potential. According to recent determinations 

 this difference may vary with the state of the surfaces of the 

 two metals between 0*7 and 1'02 volts ; and the copper is 

 negative to the zinc. 



Let the two disks be at so great a distance apart that no 

 sensible induction can take place between them. Now bring 

 them much nearer together, keeping them parallel and coaxial. 

 On account of the mutual induction called into action the 

 electric density on their opposed (inner) surfaces commences 

 to increase, and on the outer surfaces two layers of free elec- 



* For criticisms of Exuer's experiments see Beetz, Wied. Ann. xii. 

 p. 290; Hoorweg, Wied. Ann. xi. p. 133 and xii. p. 90; Julius, Wied. 

 Ann. xiii. p. 27t>; Sehulze-Berge, Wied. Ann. xv. p. 440 and xii. p. 307; 

 Von Zahn, Untersuchungen iiber Contakt-elektn'citat, Leipsic, Teubner, 

 1882 ; Ayrton and Perry, Phil. Mag. (1881) p. 43 ; Lodge, loc. cit. ; 

 Stoletow, Journ. de Physique [2] i. p. 57 ; Uljanin, Wied. Ann. xxx. 

 p. 699 ; Hallwachs, Wied. Ann. xxxii. p. 64 ; Wiedemann, Elektricitdt, 

 new edition, ii. p. 990 et seq. 



