128 ADVANCED ELECTRICITY AND MAGNETISM. 



condenser is 2.04 times as great with kerosene between the plates 

 as with air between the plates. The accompanying table gives 

 the inductivities of a few dielectrics. 



79. Dependence of capacity of a condenser upon size and 

 distance apart of plates. When the dielectric of a condenser is 

 of uniform thickness and when the metal plates are large as 

 compared with their distance apart (thickness of dielectric), 

 then the capacity C of the condenser is proportional* to a/x 

 for a given dielectric, where x is the thickness of the dielectric 

 and a is the area of the sheet of dielectric between the plates. 

 Therefore, if we choose a given dielectric, we may write 



in which G is a constant. When x is expressed in centimeters, 

 and a in square centimeters; when air is chosen as the dielec- 

 tric; and when C is expressed in farads; then the value of G 

 as found by experiment is 884 X io~ 16 . Therefore we have: 



^in farads = 884 X IO~ 16 X ^ (2) 



When a dielectric whose inductivity is k is used instead of air, 

 the capacity of the condenser is k times as great, or: 



ka 



^in farads = 88 4 X IO~ l6 X~ (3) 



in which C is the capacity in farads of a condenser of which 

 the plates are separated by a layer of dielectric x centimeters 

 thick and a square centimeters in area (between the plates), 

 and k is the inductivity of the dielectric. The meaning of a 

 may be understood with the help of Fig. 84. If there are 501 

 sheets of tin foil there will be 500 sheets of dielectric, and a will 



* It can be shown from almost purely geometrical considerations that C is 

 proportional to a/x, but it is sufficient to accept this proportional relation as tin- 

 result of experiment. The value of the proportionality factor G must be deter- 

 mined by experiment directly or indirectly. 



