Inductive Capacity of Dielectrics. 411 



plates of the standard condenser substituted in Kirchhoflf's 

 formula gives, with a minimum amount of computation, the 

 capacity of the condenser containing the specimen. By 

 determining the capacity when the specimen condenser is 

 empty and when it is filled with a dielectric, the specific 

 inductive capacity of the dielectric is determined. 



In this way the specific inductive capacity was determined 

 of several oils as follows: — Petroleum 1*99; cotton-seed oil 

 3"-00 ; olive oil 3 - 02 ; castor oil 4-49. 



The period of the electrical oscillations used in charging the 

 dielectrics was determined by a method due to J. J . Thomson*. 

 Two bare copper wires of about 20 metres length were stretched 

 parallel to each other about the room with one end of each 

 wire attached to one of two small copper disks placed close 

 to the plates of the standard air-condenser. The remaining 

 two ends of the long copper wires were left free. A sensitive 

 telephone was connected by wires of equal length to the long- 

 conductors. When one telephone terminal is attached to one 

 end of one of the long conductors and the other terminal 

 made to slide along the other conductor, the sound in the 

 telephone will pass through a series of intensities from mini- 

 mum to maximum. The linear distance between any two 

 points of the conductor giving a minimum sound is one half 

 of the wave-leneth of the electrical vibrations charoino- the 

 condenser containing the dielectric. The wave-length was 

 found to be about 9'10 metres, which corresponds to a vibration 

 frequency of 33,000,000 per second. Since it is only the 

 order of magnitude of the vibration frequency that is required, 

 and as the accuracy of this determination is necessarily un- 

 certain, this value was taken to apply to the case of all the 

 dielectrics examined. This is the more justifiable as the range 

 of these specific inductive capacities extends only from 1 99 

 to 4-60. 



This method is also applicable to the determination of the 

 specific inductive capacity of solids when large specimens nan 

 be obtained of known dimensions, and when the constants of 

 the condenser are known. In these experiments, however, 

 these requirements were eliminated by the simple device of 

 using in the condenser such a mixture of liquid electrolytes 

 that the galvanometer-needle would be unaffected by the 

 introduction of the solid being investigated. Thus the specific 

 inductive capacity of the mixture in the condenser equals that 

 of the solid. 



When the condenser-cell was filled with a mixture of 

 petroleum and castor oil of specific inductive capacity 2'32. 

 * Proc. Roy. Sue. xlvi. p. 1. 



