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LII. The Specific Inductive Capacity of Dielectrics in slowly and 

 rapidly changing Electric Fields. By Ervin S. Ferry *. 



EVER since Maxwell pointed out that the electromagnetic 

 theory of light requires a certain definite relation to 

 hold between the index of refraction and the dielectric constant 

 of a medium transmitting light, persistent attempts have been 

 made to determine the dielectric constants of various optical 

 substances, in ihe hope that such determinations would afford 

 an explanation of anomalous reflexion and refraction, and 

 the peculiarities observed when light is transmitted through 

 crystalline media that are heated or are under mechanical, 

 electrostatic, or magnetic stress. The results obtained by 

 different experimenters vary through a wide range, due largely 

 to the great difficulties in the determination of the dielectric 

 constant, and also in many cases to lack of knowledge of the 

 periodicity of the electrical disturbance employed in making 

 the measurements. While with our present limitations 

 regarding the production of measurable electromagnetic 

 oscillations of a period approaching that of visible radiance, 

 it may be impossible to obtain quantitative data bearing upon 

 anomalous reflexion and dispersion, still the piezo-optical, 

 electro-optical, magneto-optical, and pyro-optical problems 

 appear of perfectly definite solution. It was with regard to 

 the study of this class of phenomena that the present series of 

 experiments was begun. 



The object of the present investigation is the perfection of 

 a null method of determining the specific inductive capacity 

 of crystals and other optical substances that does not require 

 large masses of the substance ; that does not require the 

 dimensions of the substance to be known or the specimen to 

 be of any particular shape ; that is independent of the nature 

 of the surface ; that is sensitive to small variations of the 

 quantity measured ; in which the labour of computation of 

 results is reduced to a minimum ; and in which the periodicity 

 of the oscillations used can be easily experimentally obtained. 



Three different properties of dielectrics have been utilized 

 to determine the specific inductive capacity of substances. 

 Cavendish discovered that the capacity of a condenser depends 

 upon the medium between the plates. The ratio of the 

 capacity of a condenser having any given dielectric between 

 its plates to its capacity with air as the dielectric, was called 

 by Faraday the Specific Inductive Capacity of the given sub- 



* CoruiiHUiicated by the Author. 



