Contact with Dielectrics. 



:95 



a similar test was applied, using 150 volts. The resistance 

 calculated from the 1st min. ( + ) was 46*8 megohms. The 

 resistance calculated from the 11th min. ( — ) was 46 "5 

 megohms, a change of only 0*3 megohm against the change 

 of 0*6 in the corresponding test with 750 volts. The earth- 

 readings at the 6th min. and 16th min. were respectively 

 — 3*5 aud + 2, as against — 10*5 and +7*5 with 750 volts. 

 The earth-readings therefore are not directly proportional 

 to the applied voltages originating them. 



Table VIII. 



Mercury Electrodes (750 volts). 



Min. 



1 



Deflexion. 

 +4630 



Min. 



Deflexion. 



Min. 



Deflexion. 



19 



+3 



36 



+ 7-5 



2 



4-4589 



20 



+2 



37 



+5 



3 



+ 4548 



21 



-J-4721 



38 



+3'5 



4 



+4524 



22 



+4679 



39 



+3 



5 



+4499 



23 



+4638 



40 



+ 2-5 



6 



-]0-5 



24 



+4606 



41 



+4762 



7 



-7-5 



25 



+4565 



42 



+4729 



8 



-6-0 



26 



— 9 



43 



+4696 



9 



-5-0 



27 



-6 



44 



+ 4655 



10 



-4-5 



28 



-5 



45 



+4622 



11 



-4696 



29 



-4 



46 



-9 



12 



-4679 



30 



-35 



47 



-6 



13 



-4671 



31 



-4737 



48 



— 5 



14 



- 4630 



32 



-4713 



49 



-4 



15 



-4606 



33 



-4696 



50 



-3-5 



16 



+7-5 



34 



-4663 



51 



-4787 



17 



+5 



35 



-4639 



52 



-4762 



18 



+3-5 







53 



-4729 



i 



The maximum earth-reading is 10'5 with 750 volts. The 

 effect of this residual charge upon subsequent readings is only 

 0*23 per cent.; or in terms of resistance say 0*1 of a megohm 

 in the 42 megohms here concerned. In comparison with the 

 effects of changes of humidity, it is probably negligible. The 

 uniformity of the earth-readings in the above table is instruc- 

 tive. The ( + ) current applied at the 1st min. strikes the 

 dominant note, and gives an earth-reading which at the 6th 

 min. is — 10"5. At the 11th min. the ( — ) current is applied, 

 and gives an earth-reading which at the 16th min. is +7*5, 

 i. e., 3 divisions less than the corresponding earth-current of 

 the previous charge ; and we may assume that if the earth- 

 current at the tenth miuute had been allowed to act alone it 

 would have fallen from —4*5 at the tenth to —3 at the 

 sixteenth minute. At the 21st min. the ( + ) current is again 

 applied; the corresponding earth-current at the 26th min. 



