96 STATIC ELECTRICITY 



which the facts are only beginning to fall into order. We refer 

 the reader to the Encyclopaedia Britannica, llth ed. vol. ii. p. 860, 

 or to Gockel Die Luftelektrizitat. 



Simple methods of measuring capacity. We shall 

 describe two methods of measuring capacity merely in order to 

 make the idea of capacity more definite. Other methods will be 

 described later or may be found in laboratory manuals. 



The definition of capacity shows that it is in dimension a 

 length. Thus the capacity of a sphere, far from surrounding 

 conductors, is equal to its radius, and the capacity of two parallel 

 plates in air is equal to the area of either /4?r distance apart, when 

 the distance apart is so small that the edge effect may be neglected. 

 We may then determine the capacity in either of these cases by 

 actual measurement of the dimensions of the system. But when 

 the system is of such form or with such a dielectric that we cannot 

 determine the capacity by direct measurement of dimensions, then 

 we may use a method of comparison. 



Capacity of a Leyden jar by comparison with a 

 sliding condenser with parallel plates. Let AB, Fig. 69, 



A B 



ITI'IT'I'I'I'ITI'ITI'I 



FIG. 69. 



represent the sections of two round plates mounted on insulating 

 supports with bases sliding along a divided scale, so that the 

 plates are always parallel to each other and so that their distance 

 apart can be observed. The capacity of this "sliding condenser' 1 

 can therefore be varied at will. The method consists in charging 

 this sliding condenser and the system of which the capacity is 

 required with electrification at the same difference of potential, 

 and then varying the distance apart of the plates of the sliding 

 condenser till the charges on the two are equal. 



Suppose AB, Fig. 70, are the plates of the sliding condenser, 

 and let J be the jar on an insulating stand. Let A and the 

 outside coating of the jar be connected to earth. Let B and the 

 inside coating of the jar be connected to the positive terminal of a 

 battery whose negative terminal is earthed. Then disconnect the 

 battery from both and insulate A and the outside coating of the 



