DEFINITION --OF POTENTIAL. 



CHAPTER II. 

 ON POTENTIAL. 



19. WE shall assume, in the first place, in conformity with experi- 

 ment, that the action of two electrified bodies, of small dimensions, 

 takes place along the straight line joining them, and only varies with 

 the distance ; that it satisfies, in short, the definition of what are 

 called central forces ; and, lastly, by definition (4), that the action is 

 proportional to the product of the quantities of electricity which 

 the bodies possess. 



We shall assume, moreover, that the reciprocal action of two 

 electrified bodies of finite dimensions is the resultant of the 

 actions which would be exerted, according to the same function 

 of the distances, between the elementary masses which make 

 up the charge. 



20. ELECTRICAL FIELD. The term electrical field is applied to 

 the entire extent of the space throughout which the action of any 

 given electrical system is exerted. An electrical field is generally 

 unlimited ; it may be bounded in the case, for instance, in which all 

 the acting masses are inside an entirely closed conductor. For 

 masses whose magnitude and position are defined, the electrical force 

 at each point of the field is merely a function of the co-ordinates of 

 the point. 



The force is zero in all conductors in a state of equilibrium ; the 

 electrical field does not comprise the volumes of conductors it is 

 formed of intermediate spaces occupied by an insulating medium or 

 dielectric. 



21. LINES OF FORCE. A line of force in an electrical field is a 

 line tangential at each point to the direction of the force. Such a 

 line is obviously continuous, so long as it does not encounter acting 

 masses. 



22. DEFINITION OF POTENTIAL. Consider a system in equi- 

 librium, and suppose that all the acting masses being fixed in their 

 several positions, we move unit mass of positive electricity from 



