IDEA OF POTENTIAL. i;i 



can calculate the value of E for a given value of Q, this equation 

 will give an expression for C. 



The intensity e of the electric field at the point p in Fig. 114 



is: 



B Q 



e -5 volts per centimeter (2) 



4x x 2 



(see Art. 92). Multiplying this value of e in volts per centi- 

 meter by Ax in centimeters we get the electromotive force AE 

 along Ax. That is : 



whence, integrating, we get: 



E _BQf-*d*_BQ(j_ JL\ ,.. 



"VJU *. 4\A */ 



But C = -= according to equation (i). Therefore, from equa- 

 jii 



tion (4) we get: 



where J?i and -R 2 are expressed in centimeters, and B = 

 1.131 X io 13 . 



If R 2 is indefinitely great, then -= is negligible as compared 

 with and equation (i) becomes: 



Capacity in farads of a sphere 

 distance from all surrounding 



2 at a great 1 _ 4wRi 

 ing bodies. J ^ 



Note. Equations (5) and (6) give the values of the capacity 

 when the dielectric is air. For any other dielectric the capacity 

 is k times as great, where k is the inductivity of the dielectric. 



102. Capacity of a condenser consisting of coaxial metal 

 cylinders with air between. Let Fig. 114 represent a sectional 

 view of two coaxial metal cylinders. The inner cylinder carries 



