concerning Voltas Contact Force. 377 



If the potential of a metal is high, positive electricity tends 

 to flow away from it or out of it, and does flow if a path free 

 from obstruction is provided. If the potential of a metal is 

 low, positive electricity tends to flow into it. 



That the potential of an isolated piece of zinc is lower than 

 the potential of an isolated piece of copper is proved (I hold) 

 by the undoubted fact that a rush of electricity takes place 

 from one to the other directly they are put into contact. 

 The electricity is driven by the difference of potential : the 

 metals may be brought near and facing each other so as to 

 constitute the plates of a condenser of appreciable capacity ; 

 then let them touch, and the flow automatically occurs. If 

 potential is that which determines electrical transfer, if it be 

 the property whose gradient is electromotive intensity, the 

 natural statement about the previous potential of the two 

 metals is obvious. Before contact they were at different 

 potentials ; while they are in contact they are at the same 

 potential. The difference of potentials is due to the unequal 

 atomic forces straining at their frontier ; the equality of 

 potentials is caused by the equalising electric transfer, i. e. 

 to the production of charges which hold the still existing 

 chemical forces in equilibrium. What can be simpler than 

 this mode of expression ? 



The other mode of expression says that the metals in 

 contact are at different potentials ; meaning that the air near 

 each is at a different potential. If that is the meaning, why 

 not say so ? It is an undoubted fact. The step of potential 

 in the air is experimentally demonstrable, the only question 

 concerns the step of potential across the junction of the two 

 metals. 



To transfer a unit charge from copper to zinc in contact 

 involves no appreciable work, whether it goes through the 

 junction or whether it goes out and through the air*. To 

 transfer electricity from near the copper to near the zinc 

 involves a definite amount of work, provided that by " near *' 

 we do not mean within molecular range; and it requires the 



* I think this is the clearest form of statement. It is possible to say 

 there is a cyclical E.M.F. round a circuit like this with two different 

 paths, and that the resultant force is opposed by air resistance ; but when 

 the gradient of potential in the air is taken into account as part of the 

 E.M.F. , there is equilibrium, and the cyclical E.M.F. is zero. It is 

 merely a question as to which is the most convenient form of expression ; 

 there is no controversy in it : the question is touched on again at the end 

 of the next footnote. All that I require for my purpose is the admission 

 that no appreciable work is needed to transfer electricity across the 

 copper-zinc j unction ; while work is needed to transfer it in the air from 

 near the copper to near the zinc. 



Phil May, S. 5. Vol. 49. No. 299. April 1900. 2 D 



