168 ELEMENTS OF EEECTRICITY AND MAGNETISM. 



force of the battery is balanced by what one may perhaps call the 

 electro-elastic reaction of the dielectric DD, and the amount of 

 charge drawn out of B and forced into A is proportional to 

 the electromotive force of the battery. 



The extent to which the diaphragm DD, Fig. 104, yields is 

 measured by the amount of water q which is drawn out of B 

 and forced into A, and the yield per unit of pressure-difference is 

 a sort of coefficient of elasticity of the diaphragm. The extent 

 to which the dielectric DD, Fig. 103, yields is measured by the 

 amount of charge q which is drawn out of the plate B and 

 forced into plate A, and the amount of yield per unit of electro- 

 motive force of the battery (qjE= C) is a sort of coefficient of elec- 

 tro-elasticity of the layer of dielectric and it is called the capacity 

 of the condenser. It is important to remember that the capacity 

 of a condenser is not analogous to the cubic capacity of a vessel 

 but that it is analogous to the cubic capacity of a rubber bag, the 

 amount of water that a rubber bag will hold depends upon 'the 

 pressure. 



90. Inductivity * of a dielectric. If the diaphragm DD in 

 Fig. 104 were made of a stiff material like steel, the amount of 

 yield per unit of pressure-difference would be very much less 

 than if the diaphragm were made of a substance like rubber. 

 This is analogous to the fact that the capacity of the condenser, 

 AB, Fig. 103, with- plates of a given size at a given distance 

 apart depends upon the nature of the dielectric between the plates. 

 The quotient : Capacity of a condenser with given dielectric, 

 divided by the capacity of the same condenser with air between its ' 

 plates is called the inductivity of the dielectric. For example, 

 the inductivity of petroleum is about 2.04, that is, the capacity 

 of a given condenser is about 2.04 times as great when the dielec- 

 tric is petroleum as it is when the dielectric is air. A condenser 

 is called an air condenser, a mica condenser, a paraffin condenser, 

 etc., according to the dielectric between its plates. The accom- 

 panying table gives the inductivities of a few dielectrics. 



* Sometimes called specific inductive capacity. 



