Contact with Dielectrics. 



491 



Jn the above table it should be observed that with the 

 small load the first-minute readings are never greater than 

 the second-minute readings ; and that the apparent resistance 

 of the dielectric diminishes as the testing-voltage increases. 

 With the greater load, the first- minute readings are never 

 less than the second-minute readings, and although there is 

 a diminution of resistance with increased voltage, the effect 

 is far less marked than in the case of the small load. Increase 

 of lond is again seen to have a similar effect to increase of 

 voltage. 



At the end of these tests the dielectric sheet was tested 

 between mercury surfaces, proceeding exactly as when using 

 tinfoil electrodes. The results are shown in Table III. For 

 this purpose the dielectric sheet is held in a vertical position 

 symmetrically between two flat rings of ebonite, faced on 

 each side with soft indiarubber. Disks of iron, forming in 

 effect the jaws of a large ebonite vice, are then clamped 

 one against the outer face of each ring. Mercury is poured 

 into each of the hollow spaces between the iron disks and the 

 dielectric, through holes in the top of each disk. The 

 temperature can be conveniently read by placing a ther- 

 mometer in the mercury. 



Table III. 





Mercury Elect 



rodes. 





Volts. 



Shunt, 

 ohms. 



Deflexion., 



Deflexion, 



Megohms. 





1 min. 



2 min. 





150 



700 



325 



325 



10-5 



300 



400 



387 



386 



10-3 



450 



200 



299 



298 



10-2 



600 



200 



397 



396 



10-2 



750 



140 



350 



348 



102 



600 



200 



396 



394 



10-2 



450 



200 



298 



297 



10-2 



300 



400 



387 



385 



10-3 



150 



700 



328 



327 



10-4 



750 



140 



348 



345 



103 



150 



700 



330 



328 



10-3 



This table renders it doubtful whether the true resistance 

 of press-spahn varies greatly with the voltage, and it shows 

 that the results obtained with tinfoil electrodes may be mis- 

 leading when imperfect contact is interpreted as high re- 

 sistance of the dielectric. In confirmation of the foregoing 

 results, the sheet was left for a clay, and was then tested 

 again between mercury electrodes, the current being reversed 

 as indicated in Table IV. 



