1889.] Specific Inductive Capacity of Dielectrics. 



293 



denser, L the coefficient of self-induction in electromagnetic measure 

 of the circuit connecting the plates of the condenser, the wave-length, 

 if it is long compared with the length of this circuit, equals 2tt^ (LC). 

 Thus, if we can measure the wave-length of the vibrations executed 

 by such a system, we can find the specific inductive capacity of a 

 dielectric. For, if we determine the wave-length of the system first 

 when the plates of the condenser are separated by air, and then 

 when they are separated by a slab of the dielectric whose specific 

 inductive capacity we wish to measure, the ratio of the squares of the 

 wave-lengths will be the ratio of the capacities of the condenser in 

 the two cases, and if we know this ratio we can deduce the specific 

 inductive capacity of the dielectric interposed between the plates. 



The arrangement of the experiment was as follows : — 

 The condenser consisted of two circular zinc plates, AB, CD, 

 30 cm. in diameter ; these were supported on an insulating stand, and 

 the distance between them could be altered at pleasure. To these 

 plates wires, EF, GrH, each about 25 cm. in length, terminating in the 

 highly polished balls, F, H, were attached. The plates were also 

 connected with the poles P, Q, of an induction coil, and when this 

 was in action a succession of sparks passed between the balls F and H. 

 The periodic distributions of electricity thus produced over the plates 

 sent electrical waves down two insulated wires, each about 20 metres 

 in length, attached to the small zinc plates, L and M, placed close to 

 the plates of the condenser. 



The wave-length of the vibrations transmitted along the wire was 

 determined by the method I described in a former paper (" Note on 

 the Effect produced by Conductors in the Neighbourhood of a Wire on 



