78 



Conductors and Condensers 



[CH. m 



of area A, parallel to one another, the distance between any two adjacent 

 plates being d. If alternate plates are joined together so as to be in electrical 

 contact the space between each adjacent pair of plates may be regarded as 



\ 



FIG. 34. 



KA 



forming a single parallel plate condenser of capacity j -, , so that the capacity 



of the compound condenser is (nl)KA/4>Trd. By making n large and d 

 small, we can make this capacity large without causing the apparatus to 

 occupy an unduly large amount of space. For this reason standard con- 

 densers are usually made of this pattern. 



90. Guard Ring. In both the condensers described the capacity can 

 only be calculated approximately. Lord Kelvin has devised a modification 

 of the parallel plate condenser in which the error caused by the irregularities 

 of the lines of force near the edges is dispensed with, so that it is possible 

 accurately to calculate the capacity from measurements of the plates. 



The principle consists in making one plate B of the condenser larger than 

 the second plate A, the remainder of the space opposite B being occupied by 

 a "guard ring" C which fits A so closely as almost to touch, and is in the 

 same plane with it. The guard ring C and the plate A, if at the same 

 potential, may without serious error be regarded as forming a single plate of 

 a parallel plate condenser of which the other plate is B. The irregularities 

 in the tubes of force now occur at the outer edge of the guard ring (7, while 

 the lines of force from A to B are perfectly straight and uniform. Thus if A 

 is the area of the plate A its capacity may be supposed, with great accuracy, 

 to be 



where d is the distance between the plates A and B. 



