476 MEASUREMENT OF CAPACITY. DIELECTRICS. 



be considered as sensibly equal to unity. The specific inductive 

 capacity of a dielectric will then be determined by the ratio of the 

 capacities of a condenser, when the medium interposed is alternately 

 the dielectric in question and air (108). 



This is the method first used by Cavendish.* The ratio found 

 by experiment between the capacity of a Franklin's pane of surface S 

 and thickness e, and that of a system of two concentric spheres 

 separated by air, has always been greater than that which would 



S 

 result from the theoretical formula . These experiments lead to 



47T 



very high specific inductive capacities, 3 to 10, owing to the influence 

 of time and of phenomena of absorption. 



In a first series of researches on this subject, Faraday f compared, 

 by a division of charges, the capacities of two spherical condensers 

 of the same dimensions, one containing air, and the other a solid 

 dielectric. He thus found the following values : 



Spermaceti 1*45 Shellac 2-0 



Glass 176 Sulphur 2-24 



1065. Comparing the charges taken for the same difference of 

 potential (1034) for plate condensers of the same dimensions, 

 Gaugain brought out the influence of the duration of the charge 

 on the apparent value of the specific inductive power. With com- 

 mercial stearic acid, this power was found to be 1*3, 1*85, 2-17 and 7, 

 according as the charge was a small fraction of a second, or two 

 seconds, or a minute, or lastly, several hours. These variations 

 are very unequal for different bodies, and even the order of the 

 inducing powers depends on the duration of the charge. He ob- 

 tained, for instance : 



Charge of a Fraction of a Second. Charge of Two Seconds. 



Stearic acid 1*30 Sulphur 171 



Wax 1*50 Stearic acid 1-92 



Sulphur 1*57 Wax 2*21 



Traces of foreign substances, such as a layer of powder or of 

 moisture, render the surface of bodies better conductors, and have 

 the effect of increasing the apparent value of the specific inductive 



* CAVENDISH. Electrical Researches (published by Maxwell), p. 183. 

 t FARADAY. Experimental Researches, Series XL, 1187. 1837. 



