Dielectrics in a Steady Field of Force* 391 



its full value in a much shorter time than the period of such 

 a moving system, and a single observation of the latter would 

 suffice to determine the dielectric constant of the ellipsoid. 

 In all solid dielectrics there is. however, slow electrical 

 movement under the influence of the field within the ellipsoid 

 and a consequent accumulation of charge at each end. The 

 restoring couple, and therefore the apparent dielectric con- 

 stant, increase until the limit to molecular polarization is 

 reached. 



The advantage of the method is that the " conduction '* 

 current is confined to the separation of charge within the 

 solid symmetrically about the central section. 



An account is given below of observations made upon the 

 behaviour of various solid dielectrics, in which the total 

 polarization is expressed in terms of the apparent dielectric 

 constant. From these observations the resistivity of insulators 

 has been found in a new way. 



A difference of one part in a thousand in the dielectric 

 constant, can be readily measured by the suspended ellipsoid 

 method. On account of this sensitiveness three stages in 

 the polarization were found to be present in all the homo- 

 geneous substances examined. First a sudden very rapid 

 yield, then a slow change at a uniform rate, and finally 

 an approach to saturation. The first suggests a normal 

 slackness of charge in the dielectric molecule, the seeond 

 is probably a continuation of the polarization by the influence 

 of adjacent molecular charges, a slow linear change of the 

 nature of a simple strain. In the third stage the rate of 

 displacement ceases to be uniform, and the polarization 

 reaches a maximum which is shown in the paper to be 

 independent of the intensity of the polarizing field within 

 the specimen. 



According to this " three stage " theory, the quantity 

 passing in the instantaneous charge or discharge of a con- 

 denser with a solid dielectric, involving as it does the first 

 stage only, should be proportional to the voltage ; the total 

 residual charge corresponding to the later stages, the ap- 

 parent dielectric constant of which is independent of voltage, 

 should also be proportional to the voltage, as shown by Hop- 

 kinson* to be the case. Dielectric hysteresis loss should 

 decrease with rise of frequency, on account of the smaller 

 range possible in the second stage, which agrees with the 

 experimental results of Threlfallf. 



* Roy. Soc. Phil. Trans, vol. clxvii. partii. pp. 599-620 (1877). 

 t R. Threlfall, "On the Conversion of Electric Energy in Dielectrics," 

 part iii. Physical Review, vol. v. p. 6">. 



