18 ELECTRICAL ENGINEERING 



The permeance % is directly proportional to the cross-sectional 

 area of the path and to its dielectric constant and is inversely 

 proportional to the length of the path. It is usually difficult to 

 calculate the permeance directly, since the section of the path 

 varies throughout its length. In such cases the dielectric flux \f/ 



or the electric charge Q = ~- is assumed and the potential differ- 

 ence E calculated as in the preceding examples. 



B 

 FIG. 13. Permeance of the path between a conductor and a plane.. 



The permeance is then 

 and the capacity is 



r 



-- 



thus the capacity is equal to the dielectric permeance of the path 

 divided by 4 TT. 



20. Condenser. An electrical condenser is ail arrangement 

 of conductors which is capable of storing a large quantity of 

 electricity, or which has a large capacity. It is one in which a 

 large flux is produced when a given potential difference is applied 

 to its terminals. 



21. Capacity of Condensers. The capacity of a condenser is 

 measured by the amount of charge necessary to raise its potential 

 by unity, and is therefore equal to the positive charge on it at 

 any time divided by the potential difference between its terminals; 



