concerning Volt a 9 s Contact Force. 367 



nected both metallically and dielectric 'ally ', that is if they are 

 insulated from each other except at one point where the 

 intervening dielectric is swept away, we have the normal 

 conditions appropriate for a display of the Volta effect : that 

 is, a gradient of potential in the dielectric between them. 



The safest and clearest mode of expression is what I have 

 used : the Volta effect consists in an opposite charge acquired 

 by dry zinc and copper while in metallic contact, a charge 

 which results from an E.M.F. of value depending on the con- 

 dition of their outer surfaces, and controlled solely by this 

 E.M.F. and the electrostatic capacity. 



I here avoid any statement involving the term metallic 

 potential, because it is open to misconception owing to variety 

 of definition. Some people prefer to say that zinc and copper 

 in contact are at different potentials, whereas 1 prefer to say 

 that they are at the same potential so long as they are in con- 

 tact : but no one can deny that they are oppositely charged. 



Some people, again, prefer to say that they are oppositely 

 charged by reason of an E.M.F. at the junction, which 

 keeps them at different potentials : I prefer to say that their 

 charges are distributed so as to enable them to remain at the 

 same potential, in spite of a difference in the specific Volta 

 force existing at the dielectric boundary of each; in other 

 words, that the E.M.F. is at their surface ; but again, no 

 one can deny that there exists in the dielectric between 

 them a gradient of potential, the natural result of their 

 opposite charges ; which same gradient can cause a charged 

 electrometer needle to move, even without the magnification 

 of potential caused by separation. In fact the magnification 

 of potential by separation does not (within limits) increase the 

 gradient, for the potential rises as the capacity diminishes, 

 and so remains in direct proportion to their distance apart, 

 so long as that distance is moderate. 



Nor can anyone deny that oppositely charged bodies, or 

 surfaces between which there is a gradient of potential, attract 

 each other. Hence, indirectly, a piece of zinc attracts a piece 

 of copper with which it is at any point in contact ; and the 

 attraction increases as the square of the potential gradient, and 

 therefore as the square of the nearness together of the opposed 

 or condenser surfaces. 



Signor Majorana (Phil. Mag. Sept. 1899) has designed 

 some simple methods for displaying this attraction, one of them 

 the same as a method I had used for displaying the attraction 

 of two metals exposed to Hertz radiation just before cohesion; 

 viz., a fine metallic fibre hanging in front of a polished facet 

 on a knob, the fibre and its image being watched through a 



