242 Sig. Quirino Majorana on the Contact Theory. 



Helmholtz, in his Erhaltung der Kraft *, uses an argument 

 which may be stated thus : — Suppose that any number of 

 pieces of metal of any form and nature are put in connexion 

 with the earth and occupy definite positions. They are then, 

 electrically, in the so-called neutral state. By the principle 

 of conservation of energy, when any two of these pieces of 

 metal are directly connected together after having removed 

 the wires connecting each to earth, no flow of electricity can 

 take place unless the positions of the pieces be changed, 

 thereby altering their electrical capacity. If this were not 

 the case it would only be necessary to put the metals into 

 contact with each other and the earth alternately, in order to 

 obtain a flow of electricity each time ; this is impossible 

 since the establishment of a metallic communication does not 

 constitute work. 



On developing the idea of Helmholtz we arrive at the 

 conclusion that metals, when connected to earth, are sur- 

 rounded by an electric layer, the potential of which varies 

 from one metal to another. By establishing a, metallic 

 communication between any two of them the distribution of 

 electricity in the system is unaltered, and according to 

 Volta's theory the two metals, when metallically connected, 

 are at different potentials. 



Experimentally, w r e have therefore : — 



[a) An electrometer (which is itself made of metallic 

 parts) does not exhibit any electric charge if, after 

 having been put to earth, it is connected with one of 

 the elements of a dry copper-zinc couple. In this opera- 

 tion no alteration is made in the capacity of any con- 

 ductor. 



(h) If we wish to study the electricity set free by the 

 electromotive force of contact, it is necessary to alter the 

 capacity of the system of two dissimilar metals. 



The preceding considerations will show that in teaching 

 the subject we do not generally state clearly and simply the 

 fundamental conception of the phenomenon discovered by 

 Volta. The metallic contact of the two metals is considered 

 essential, and the beginner often supposes that when (his 

 contact has not been made, the two metals, after having been 

 connected to earth, have the same potential. 



Although it is certain that two metals in contact are at 

 different electric potentials, the idea that this difference is 



* Translated in Taylor's ' Scientific Memoirs,' new series (1853) (Nat. 

 Phil.), p. 114. 



