402 Prof. W. M. Thornton on the Polarization of 



6. Measurement of Specific Resistance. 



The fact that the rate of change of the effective dielectric 

 constant is uniform for some time after the tield is applied, 

 provides a method of measuring the specific resistance of a 

 dielectric during polarization as distinct from that using the 

 " time of relaxation." 



Whatever the nature of the electrical movement during 

 the polarization of an insulated ellipsoid maybe it is in effect 

 a current, the density of which is the rate of change of 

 the total polarization in unit volume. The field within the 

 ellipsoid is given by F^F/fl + zeN), where k = (K — l)/4-n\ 

 The polarization is then KF l5 and writing the apparent specific 

 resistance p, we have 



d fTr „^ F 



P 



The resistance measured in this way differs, though not to a 

 marked extent, from that found by the passage of a steady 

 current through the dielectric. 



It may be said that in view of the facts of slow polarization 

 the measurement of the current after one minute's electrifica- 

 tion is meaningless as an indication of the resistance of the 

 insulation. A large part of the current is polarization change. 

 The leakage method of measuring the resistance of insulators 



CD J~> 



is also complicated by this slow depolarization of the dielec- 

 tric and greatly influenced by the time of charge. 

 We have from the above 



dt^=~ < 3 > 



^^l + Z^W (4) 



from which p may be found at any stage of the polarization *. 

 Since the measurements are in electrostatic units, p is also, 

 and in electromagnetic measure is 



= v 2 {±7r + (K-l)N\/dK/dt . 10 9 ohms per cm. cube, . (5) 



where v is the ratio of the electromagnetic to the electrostatic 

 unit of charge. 



The following table gives values of the specific resistance 

 found in this way from the observed rates of change of fig. 2. 

 The indiarubber and gutta-percha were not specially pre- 

 pared, for cable work. 



* The statement of this section has been modified at the suggestion of 

 Dr. T. H. Havelock, to whom the author is indebted for helpful criticism 

 throughout the work. 



