Sig. Quirino Majorana on the Contact Theory. 247 



tricity are formed, positive on the copper and negative on the 

 zinc. If the two disks be again connected to earth while in 

 this position the charges on the outer surfaces are removed by 

 the conductors used. 



When the two disks are replaced in their former positions 

 the electric density on the inner surfaces begins to diminish, 

 and a quantity of electricity (from each disk) in excess of 

 that which the so-called electromotive force of contact would 

 maintain, flows through the conductors to earth. This 

 quantity of electricity is exactly equal, but of opposite sign, 

 to that which is liberated during the approach of the two 

 disks. If the disks are brought near each other and put to 

 earth, and are then metallically connected, we again obtain 

 the same result, because the electromotive force of contact 

 prevents the neutralization of the positive charge of the 

 zinc and the negative of the copper, but does not hinder the 

 neutralization of the free charges developed during the 

 approach. This alone happens when the disks are put to 

 earth, because even then electromotive forces exist at the 

 points of contact. 



All this follows as a simple consequence of the contact- 

 theory, and may be expressed by the following laws : — 



(a) Dissimilar conductors (not electrolytes) put into commu- 

 nication with the earth acquire different potentials depending 

 on the nature of each conductor. 



(b) Whenever two dissimilar conductors, after having been 

 discharged to earth, approach each other without being brought 

 into contact, they acquire free electric charges which may be 

 removed by any conductor (not an electrolyte) whether put 

 to earth or insulated, but in the latter case the conductor 

 must be of large capacity relatively to that of the disks 

 experimented on. 



These approach-charges are opposite in sign to those obtained 

 in the ordinary Volta experiment ; so that zinc brought near 

 to copper is charged negatively, and copper brought near to 

 zinc becomes positively charged*. 



(c) Whenever two conductors of different metals (placed 

 sufficiently near each other) are separated, after having been 



* Note that this mode of expression is adopted for shortness, but is not 

 rigorously exact. In reality, a plate of zinc brought near to one of copper 

 if connected to an electrometer gives the indication of a negative charge ; 

 and thus it may be said briefly that the zinc becomes negatively charged, 

 but really the negative charge is not formed alone ; an equal positive layer 

 remains bound on the disk as long as it retains its position. In saying, 

 therefore, zinc is charged negatively, we mean that zinc connected to an 

 electrometer gives the indication of a negative charge. 



