6 ELECTRIC LIGHTING. 



The potential of a source of electricity* is related to the 

 tension ; but, being applied to the electrical actions them- 

 selves, it may represent the tension under more denned 

 conditions, which admit of numerical expression. It may 

 be roughly defined by saying that it is to electricity what 

 temperature is to heat ; it is, in a manner, the quality of the 

 electricity, and this properly has relation to the quantity. 

 The notion of the potential of an electrified conductor 

 necessarily originated from the study of the conditions of 

 electrical equilibrium, according to the laws discovered by 

 Coulomb. For the existence of these conditions of equi- 

 librium it is necessary that the resultant of the attractive and 

 repulsive forces acting upon an interior point should be nil; 

 but it does not therefore follow that the action of the elec- 

 tricity spread over the surface of a conductor should also be 

 nil; and it is the mathematical expression representing this ac- 

 tion with the zero resultant that is called the electric potential. 



The electrical intensity represents the magnitude of the 

 effect produced by the electro-motive force, that is to say, 

 the force of the current ; it is, therefore, always in proportion 

 to the quantity of electricity circulating in the conductor, and" 

 /"/ must depend upon the amounts jointly of the electro-motive 

 force and of the resistance offered by the conductor to the motion 

 of the fluids. Ohm has shown that its value may be expressed 

 by the ratio of the electro-motive force to the resistance of 

 the conductor, and this supposes that it is directly propor- 

 tional to the electro-motive force, and inversely proportional to 

 the resistance. Again, Joule has shown that the heat de- 

 veloped by an electric current is proportional to the time, to 

 the resistance of the circuit, and to the square of the quantity of 

 electricity which passes through the circuit in a given time. 



* Abria, in an interesting paper on the Elementary Theory of Potential, 

 thus defines it : " Let us call the Potential of an electrified body the indi- 

 cation of a torsion balance, in which the movable ball and the fixed ball 

 are two small equal spheres, which, being in contact, have been connected 

 with the. body by a long and thin wire." 



