414: Specific Inductive Capacity of Dielectrics. 



of refraction, in accordance with the electromagnetic theory. 

 The value of the dielectric constant in the two directions, 

 however, is nearly twice as large as the square of the corre- 

 sponding index of refraction. 



The values in the preceding table show that in the case of 

 every substance examined, there is a difference of the dielectric 

 constant for rapidly changing and slowly changing fields. 

 It is to be expected that the amount of this difference should 

 be proportional to the electrical absorption of the substance. 

 If electrical absorption is due to a heterogeneous structure of 

 the medium, as developed in the theories of Clausius and 

 Maxwell, the minimum amount of electrical absorption would 

 be exhibited by crystals; and consequently, the difference in 

 the value of the dielectric constant of a crystal for rapidly 

 changing and slowly changing fields would be less than that 

 of an amorphous substance. Rowland and Nichols * have 

 measured the electrical absorption of several substances, and 

 find crystals to exhibit very much less absorption than amor- 

 phous substances. Thus is explained the fact that the dielectric 

 constant of quartz suffers less change than the other substances 

 examined when subjected to fields of various periodicities. 

 For the same change of periodicity of charge, ebonite shows 

 the greatest difference in specific inductive capacity of any 

 of the substances examined. The insulating oils give differ- 

 ences nearly equal to each other. 



It is interesting to note in this connexion the behaviour of 

 some plates of quartz that in polarized light showed in one part 

 of the section beautiful Airy's spirals. With these specimens 

 it was difficult to obtain the specific inductive capacity, because 

 the change in position of the specimen with respect to the plates 

 of the oil condenser gave slightly different values of the 

 constant being determined. But this much is certain, that 

 the difference in the specific inductive capacity in rapidly 

 changing and slowly changing fields is greater in the part of 

 the crystal that shows Airy's spirals in polarized light, than is 

 this difference in crystals that do not exhibit Airy's spirals. 

 This observation is in accord with Maxwell's theory that 

 electric absorption is produced by a laminated structure of 

 the dielectric. 



Physical Laboratory, 

 University of Wisconsin. 



* Phil. Mag. [5] xi. p. 414 (1881). 



