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III. A Method for Comparing the Values of the Specific 

 Inductive Capacity of a Substance under Sloidy and Rapidly 

 Changing Fields : Results for Paraffin and Glass. By 

 Edwin F. Northrup*. 



[Plate III.] 



MAXWELL'S electromagnetic theory of light entails the 

 relation that the square of the index of refraction of 

 a substance, for infinitely long waves, shall equal its specific 

 inductive capacity. But as the refractive index has only 

 been obtained for very short waves (excepting some experi- 

 ments upon the refractive index of a few dielectrics for electric 

 waves), it is not unexpected that this relation should not hold 

 in the case of many substances whose specific inductive 

 capacity has been determined in steady or very slowly 

 alternating fields. It is to be expected, however, if the field 

 of force under which the value of K is obtained could be 

 made to alternate at a rate which would produce waves com- 

 parable in length to the waves with which the refractive index 

 is determined, that the agreement of K and /j, 2 required by 

 the theory would be obtained. The experiments of Professor 

 Hertz upon electrical oscillation have suggested certain 

 methods for determining the specific inductive capacity under 

 fields changing at a rate which approaches that required. No 

 research, however, aiming at a direct comparison of the values 

 of K under slowly changing and oscillating fields, which is as 

 fully conclusive as could be desired appears to have been 

 made. The objects held in view in the following investigation 

 were : — 1st. To perfect a method which will enable the specific 

 inductive capacity of solids (for which substances the value 

 of K departs most from the required relation) to be deter- 

 mined under fields of force varying slowly or rapidly as 

 desired ; and 2nd. To employ the method on one or two sub- 

 stances, to learn if the agreement with the theory is closer in 

 the latter than it is in the former case. With the method 

 under consideration this comparison may be made with the 

 same apparatus and under conditions similar in all essential 

 features. 



Description of Apparatus. 



Fig. 1, PI. III., gives in elevation the apparatus employed. 

 Fig. 2 in plan. The dimensions of the parts are given in 

 the diagram in centimetres. The following are its essential 

 * Communicated by the Author. 



