Capacity of Solids under Varying Fields. 87 



as follows: — Using the nearly correct value of K just obtained, 

 we have for the capacity of the side AB, taking d 4i = 0, 



For the side AC, 



c = 4 4 < 19 > 



Equating the right-hand members of (18) and (19), we 

 obtain 



D= < *^ (20) 



This value of D, then, makes the capacities of the two sides 

 the same, within the limits in which K is known. A still 

 closer value of K may now be obtained as before. Two 

 adjustments of the apparatus are sufficient for an accurate 

 determination. If the adjustment is exactly right a and </ 

 cancel out in expression (3), and hence 



K= dl . 



U>2 ~~ W'4 



Readings should now be taken with the different values given 

 to d±. The closeness of the agreement of the values of K 

 then obtained furnish a test of their accuracy. 



To obtain, next, the value of K under oscillating fields we 

 proceed as follows : — The two balls, n and p f are approached 

 so that sparks can pass between them, the distance being- 

 chosen such that the best conditions may exist for oscillations 

 takino- place. The movable plates are then connected to 

 the primary of the transformer, and the spark-gap to the 

 secondary. Great care should be taken that the primary 

 leads are symmetrically arranged and of the same length, 

 and that no conductor be near them. Carelessness on this 

 point led for some time to conflicting results. Waves of 

 energy should reach the transformer along both leads at the 

 same instant, which they will not do if the leads are of un- 

 equal length. The plates are set by means of the spark-gap, 

 as in the case of slowly changing fields. If the value of K 

 comes out much smaller than in the case of slow fields, 

 another adjustment of the apparatus may be necessary. This 

 being made, another determination can be made for slowly 

 varying fields, using formula (8), which for small differences 

 in the capacities of the two sides gives results correct within 

 the limits of other necessary errors. Thus, after the appa- 

 ratus has been correctly arranged, a comparison by this 



