85-89] 



The Leyden Jar 



77 



PRACTICAL CONDENSERS. 

 Practical Units. 



87. As will be explained more fully later, the practical units of 

 electricians are entirely different from the theoretical units in which we 

 have so far supposed measurements to be made. The practical unit of 

 capacity is called the farad, and is equal, very approximately, to 9 x 10 11 times 

 the theoretical c.G.S. electrostatic unit, i.e., is equal to the actual capacity 

 of a sphere of radius 9 x 10 n cms. This unit is too large for most purposes, 

 so that it is convenient to introduce a subsidiary unit the microfarad 

 equal to a millionth of the farad, and therefore to 9 x 10 3 c.G.S. electrostatic 

 units. Standard condensers can be obtained of which the capacity is equal 

 to a given fraction, frequently one-third or one-fifth, of the microfarad. 



The Leyden Jar. 



88. For experimental purposes the commonest form of condenser is the 

 Leyden Jar. This consists essentially of a glass vessel, bottle-shaped, of 

 which the greater part of the surface is coated 



inside and outside with tinfoil. The two coatings 

 form the two plates of the condenser, contact with 

 the inner coating being established by a brass 

 rod which comes through the neck of the bottle, 

 the lower end having attached to it a chain 

 which rests on the inner coating of tinfoil. 



To form a rough numerical estimate of the 

 capacity of a Leyden Jar, let us suppose that the 

 thickness of the glass is ^ cm., that its specific 

 inductive capacity is 7, and that the area covered 



with tinfoil is 400 sq. cms. Neglecting corrections required by the irregu- 

 larities in the lines of force at the edges and at the sharp angles at the 

 bottom of the jar, and regarding the whole system as a single parallel plate 

 condenser, we obtain as an approximate value for the capacity 



K.A. 

 j electrostatic units, 



O 



FlG 33 



in which we must put K=l, A = 400 and d = ^. On substituting these 

 values the capacity is found to be approximately 450 electrostatic units, 

 or about - microfarad. 



Parallel Plates. 



89. A more convenient condenser for some purposes is a modification of 

 the parallel plate condenser. Let us suppose that we arrange n plates, each 



