ELECTRIC CHARGE. THE CONDENSER. 169 



TABLE. 



Inductivities of various substances. 



Glass 3-10 



Sulphur 2.24-3.84 



Vulcanite 2.50 



Paraffin 1.68-2.30 



Rosin . . . . 1.77 



Wax . . 1.86 



Shellac 2.95-3.60 



Mica 4-8 



Quartz . 4.5 



Turpentine 2.15-2.43 



Petroleum 2.04-2.42 



Water 73-9 



The inductivity of a dielectric is determined by measuring the 

 capacity of a condenser first with air between its plates and then 

 with the given dielectric between its plates. See Chapter X. 



91. Dependence of the capacity of a condenser upon size and 

 distance apart of its plates. Using a ballistic galvanometer as 

 explained in Art. 88, it may be shown experimentally * that the 

 capacity C of a condenser, an air condenser, for example, is 

 proportional f to the area a of one of its plates, and inversely 

 proportional to the distance x between its plates ; that is, C is 

 proportional to a\x, so that we may write 



in which C is the capacity of the air condenser, a is the area 

 of one plate (sectional area of the dielectric), x is the distance 

 between the plates, and ijB is a constant. 



When C is expressed in farads, a in square centimeters, and 

 x in centimeters, then the value of ijB, as determined by 

 experiment is 884 x io~ 16 , so that equation (62) becomes 



ka 



SM*-* 884.x io- 16 x - (63) 



/ x 



* Indeed it may be shown from geometrical considerations that C must be pro- 

 portional to ajx ; the value of the proportionality factor must however be determined 

 by observation. It is possible to calculate the value of IJB from the observed 

 velocity of light as explained in Art. 146. 



f When a is large compared with x, the non-uniformity of the electric field near 

 the edges of two parallel oppositely charged plates is negligible, and it is ignored 

 throughout this discussion. 



