Dielectrics in a Steady Field of Force, 



401 



It will be noticed that gutta-percha, upon which so much 

 depends in the working of submarine cables, has the highest 

 values o£ all the substances examined. It is at least possible 

 that some of the difficulties of cable working arise from this 

 cause. If by mixing a suitable insulator of lower dielectric 

 constant with it, or by some system of grading, retaining at 

 the same time the impermeability to water, the effective 

 constant could be reduced, the rate of signalling might be 

 accelerated. 



Curves of the above rates of change are drawn in fig. 2 



Fig. 2. — Rate of Polarization of Dielectrics in Steady Fields. 



I 2 o 4- 5 ^ ~ 8 <? 10 



MINUTES IN FIELD 



for the first ten minutes of electrification, and it will be seen 

 that the features of a rapid rise at the start and a straight 

 line change following it are present in all but sealing-wax, 

 a mechanical mixture. The linear change in most cases lasts 

 for several hours. In the case of fused quartz the polariza- 

 tion is steady in half an hour, and no increase was observed 

 after several days in the field. This rapid polarization is 

 possibly the reason for its use as an insulator in electroscopes. 



The initial rush is in almost every case over within a minute. 

 This may explain why observations of cable resistance are taken 

 after this interval. 



Phil Mag. S. 6. Vol. 19. No. 111. March 1910. 2 D 



