Inductive Capacity of Dielectrics. 407 



capacity of a dielectric is determined. Timing* - has de- 

 veloped the method into one requiring only the measurement 

 of the capacity of an air-condenser. In this method a Hertzian 

 resonating circuit is placed parallel to, and at a short distance 

 from, a similar exciting circuit connected to an induction-coil. 

 By changing the capacities the two circuits can be put into 

 resonance with each other, resonance being determined by 

 noting the maximum deflexion of a dynamometer in the 

 secondary circuit. In determining the dielectric constant of 

 an electrolyte, a sample is placed between the plates of the 

 condenser in the secondary circuit and the capacity of the 

 primary circuit changed till resonance is attained. Then 

 replacing the condenser containing the electrolyte by an air- 

 condenser, resonance is again secured by changing the distance 

 separating the plates of the latter. This gives the capacity 

 of the condenser containing the electrolyte. By knowing the 

 capacity of the same condenser when empty the specific 

 inductive capacity of the electrolyte is determined. This 

 method is probably the most rapidly and most easily operated 

 of any of equal accuracy yet described. 



The method now to be described consists in getting two 

 electrical systems of equal self-induction in resonance with a 

 third system in which rapid electrical oscillations are set up by 

 means of an induction-coil and spark-gap. The periods of the 

 electrical oscillations in the two secondary circuits will therefore 

 be equal to each other, and hence the capacities in these 

 two circuits will be equal. The capacities in one of the two 

 secondary circuits is produced by an adjustable air-condenser, 

 and the capacity in the other circuit consists of a condenser 

 with plates separated by a fixed distance. By securing 

 resonance of the two secondary circuits with the primary, 

 first when the empty condenser with fixed plates is in one of 

 the secondary circuits, and then again when a dielectric is 

 inserted between its plates, the specific inductive capacity of 

 the dielectric is determined by simply computing the capacity 

 of the adjustable air-condenser in the two cases. 



The method and apparatus employed may be rendered 

 clearer by reference to fig. 1. mh is a rectangle of about 

 70 centim. per side, connected to an air-condenser c'", induction- 

 coil, and spark-gap as shown. This constitutes the primary 

 circuit. The secondary circuits consist of two rectangles of 

 equal dimensions, pb and gd, connected respectively to a 

 standard air-condenser d, and the condenser with fixed plates 

 c". In order to determine when the two secondary circuits 



* Phys. Rev. ii. p. :J5 (1894). 



