CONCLUSIONS AS TO THE DISTANCE OF THE ATOMS. 179 



each of the contacts, is a constant quantity, independent of the other 

 bodies constituting the chain ; and the difference of potential between 

 the two ends of the chain, is the algebraical sum of all the electro- 

 motive forces due to each of the contacts. 



When these two ends are joined by a conductor, no new elec- 

 tromotive forces of contact are introduced ; a permanent flow of 

 electricity is set up in the circuit, where it is maintained by the 

 energy of the chemical actions. This is the principle of electrical 

 piles or batteries. 



191. CONCLUSIONS RELATIVE TO THE DISTANCE OF THE 

 ATOMS. When two metals are in contact, the existence of an 

 electromotive force implies the formation of two electrical layers 

 of opposite signs separated by a finite distance, and these layers 

 must be localised in the two metals respectively. If we knew the 

 electromotive force, we could determine the distance between the 

 layers, by measuring the absolute charge on the two plates when they 

 are separated after having been in contact ; for if V denotes the 

 electromotive force we have 



= L X 

 4?r m 



It is almost impossible to make the experiment in this form, for 

 the charges which the plates retain depend solely on the capacity 

 of the system at the moment the separation is effected ; and this 

 capacity is in general only a very small fraction of the original 

 capacity, since contact cannot be broken simultaneously over the 

 whole extent of the surface. 



Sir W. Thomson, however, by a series of ingenious reasonings, 

 has been able to show what must be the lower limit of the distance 

 of two electrical layers. 



The expression for the electrical energy of the two plates in 

 contact is 



, 

 S-n-e 



and this energy represents the work necessary for separating the two 

 plates. This conclusion may be verified in another manner. If a- 

 is the electrical density of each of the layers, we have 



V 

 471-0-=-, 



N 2 



