366 ELEMENTS OF ELECTRICAL ENGINEERING. 



bodies A and B. If the bodies A and B are in the form of 

 metal plates separated by a thin layer of dielectric, their electro- 

 static capacity is large. Such an arrangement is called a con- 

 denser. Condensers of large capacity are made by 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. 



Units of capacity. A condenser is said to have a capacity of 

 one farad when one coulomb of charge is drawn out of one plate 

 and forced into the other plate by an electromotive force of one 

 volt; C in equation (11) is expressed in farads, when q is ex- 

 pressed in coulombs and E in volts. The farad is an enormously 

 large capacity as compared with capacities ordinarily met with in 

 practice, and the microfarad (one-millionth of a farad) is frequently 

 used as a unit. 



11. Inductivity of a dielectric. The capacity of a condenser 

 with plates of given size and at a given distance apart depends 

 upon the dielectric. The quotient : capacity of 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 dielectric is petroleum as it is when the dielectric is air. 

 A condenser is called an air condenser, a mica condenser, or a 

 paraffine condenser, according to the dielectric used between the 

 plates. The following table gives the inductivity of a few 

 dielectrics : 



TABLE. 



INDUCTIVITIES OF VARIOUS SUBSTANCES. 



Glass 3 to 10 



Sulphur 2.24103.84 



Vulcanite 2.50 



Paraffine 1.68 to 2.30 



Rosin 1.77 



Wax , . 1.86 



Shellac 2.95 to 3.60 



Mica 4 to 8 



Quartz 4.5 



Turpentine 2.15 to 2.43 



Petroleum 2.04 to 2.42 



Water 73 to 90 



