ELECTRIC CHARGE. THE CONDENSER. 



I6 5 



of stress or strain being somewhat as shown in the figure. The 

 expanded cavity is analogous to a positively charged body, the 

 contracted cavity is analogous to a negatively charged body, the 

 stressed condition of the rubber or jelly is analogous to the elec- 

 tric field between two charged bodies and the pressure-difference 

 of the pump is analogous to 

 the electromotive force of 

 the battery in Fig. iO2a. 



88. Electrostatic capac- 

 ity. The condenser. The 



amount of charge q which 

 flows out of B and into A, 

 Fig. iO2a, when the battery 

 is connected is proportional 

 to the electromotive force 

 of the battery. Therefore 

 we may write 



q=CE (6l) Fig. 102b.' 



in which q is the charge that is drawn out of B and forced into 

 A, in Fig. iO2a, by a battery of which the electromotive force is 

 , and C is a constant depending upon the size and shape of 

 A and B and upon the nature of the intervening dielectric. 

 This quantity C is called the electrostatic capacity or simply the 

 capacity of the pair of bodies A and B. If the bodies A and 

 B are in the form of flat plates of metal separated py a thin layer 

 of dielectric their electrostatic capacity is large. Such an arrange- 

 ment is called a condenser. Condensers are usually made of 

 sheets of tin foil separated by sheets of waxed paper or mica. 

 The Leyden jar is a condenser made by coating the inside and 

 outside of a glass jar with tin foil. 



Measurement of capacity. The simplest method of measuring 

 the capacity of a condenser is to charge the condenser by a bat- 

 tery of known electromotive force E and then measure the 



