Insulating Properties of Indiarubber. 523 



at all appeared to have taken place in experiments (1) and 

 (2), but in (3) the samples of No. 2 rubber showed a slight 

 discoloration on the surface, similar to that observed before 

 with this rubber. 



17. From the above experiments it appears that Para 

 rubber, which is the kind generally used in the manufacture 

 of cables, is more easily attacked by ozone or other gas 

 generated by the action of alternating stress on the air, than 

 is the rubber called No. 1. This decomposition has, how- 

 ever, only been detected when there has been comparatively 

 free access of air to the rubber. In the case of the rubber 

 called No. 1, although in the condensers F and G it was 

 very severely stressed, being kept for 3000 hours under an 

 alternating stress having a maximum value of 50,000 volts 

 per cm., no deterioration in insulating properties could be 

 detected ; but the fact that this rubber becomes harder and 

 rather brittle on keeping makes it unsuitable for cable-work. 



18. The principal conclusions which appear to be justified 

 by the foregoing experiments are : — 



(1) That in the case of the okonite cable the current 

 during the first hour's electrification increases as the charging 

 pressure increases. 



(2) That this does not necessarily prove the dielectric not 

 to follow Ohm's Law, since the true conduction-current must 

 be considerably less than the total current unless electrifi- 

 cation is continued for very lengthened periods. 



(3) That in the case of the okonite cable and the paraffin- 

 paper condenser, the charging current is a power function of 

 the time reckoned since electrification commenced, and on 

 this may be superimposed a current of true conduction. 



(4) That for the okonite cable, the mica, and the paraffin- 

 paper condensers, the discharging current is a power function 

 of the time which has elapsed since discharge began ; the 

 currents at any particular charging pressure but varying 

 time of charge being given, in the case of the okonite cable 

 by equations of the form 



-«<#•• 



where x— X=K 2 T — z ; tetime in seconds since discharge 

 commenced ; T = time of charge in seconds ; K, K 1? K 2 , K 3 , 

 and z are constants. 



(5) That the only deterioration in insulating properties of 

 indiarubber due to alternating pressure was in the case of 

 pure Para, and appeared to be due, not to any effect of 

 alternating stress on the rubber itself, but to chemical action 

 taking place between the rubber and gases produced by the 

 action of alternating stress on the air. 



