INTRODUCTORY. 



If q. 2 = 1, the work done is 



+ 0) 



and is called the Potential of $i at a distance n, and is usually 

 denoted by the letter V. 



In the same way the potential at a point distant n, r^ r 3 

 from any system q\ y q%, q 3 of electric charges in air is given by 



. (10) 



When the electric force does work in removing a positive 

 unit of electricity from a point to an infinite distance away, V 

 is reckoned positive ; but when a positive unit of electricity is 

 removed from the point to an infinite distance away work is done 

 against the electric force by some external agent, V is reckoned 

 negative. 



When a quantity of electricity is placed in an electric field, 

 the force acting on it tends, therefore, to move it from places of 

 high potential to places of low potential. 



The surface of an insulated conductor has every point at the 

 same potential, since, if such were not the case, any electricity on 

 its surface would flow from places of high potential on its surface 

 to places of low potential until equilibrium was established. 



Such a surface is called an Equipotential Surface, 

 and the value of the potential at any point of the surface is called 

 the potential of the conductor. The potential of the earth is taken 

 arbitrarily to be the zero of potential. 



1O. The potential of an insulated conductor depends not only 

 on its size, form, and the quantity of electricity on its surface, 

 but also upon its position relative to neighbouring conductors. 

 An arrangement consisting of two conductors, one of which is 

 insulated and the other (usually, but not necessarily) connected 

 to the earth, is called a Condenser. 



The quantity of electricity which must be given to the insulated 

 conductor in order that the difference of the potentials of the 

 two conductors (or coatings, as they are sometimes called) may be 

 unity, is called the capacity of the condenser. 



Capacity. The capacity of a single conductor is the charge 

 necessary to raise its potential from zero to unity. 



The C.G.S. unity of capacity is possessed by a condenser whose 



