292 Rood — Experiments on High Electrical Resistance. 





Gutta percha. 



Sealing wax, 



1st stroke 



1' 20" 



2' 2" 



2d " 



2' 9" 



5' 



3d 



2' 25" 



no stroke. 



On this day there was 30 per cent of moisture in the air. 



In all the cases thus far examined the substances in question 

 conducted sufficiently well to affect the electrometer arranged 

 with a medium sensitiveness, but there were other insulators 

 having still higher resistances, at least in dry weather. 



Ebonite. — One of the most interesting of these is ebonite or 

 hard rubber. It being very doubtful according to my previous 

 experiments whether ebonite conducted electricity at all in dry 

 weather, a strip of this material, 10 cm long and l*5 cm broad, 

 was provided with tin-foil armatures and then tested with the 

 Bohnenberger electroscope for accidental charges, and after- 

 wards connected for half an hour with a jar charged positively 

 to 12,000 volts. It was then removed from the jar, the tin- 

 foil discharged, and passed over the Bohnenberger electroscope. 

 It w T as found to be negatively charged for about 17 cm from the 

 jar end, the potential being a little under 110 volts, and gradu- 

 ally rising near the jar end, probably not much above 200 

 volts. This negative charge, I take it, is probably due to a 

 separation of the ions, owing to the prolonged inductive action 

 of the strong field. The final result is, that if the ebonite 

 conducted at all in the regular way, such conduction was 

 masked by the inductive effects. 



Gutta percha. — Exactly the same experiment was made 

 with a similar strip of gutta percha : it showed a charge of 

 positive electricity that extended 16 cm from the jar-end of the 

 strip ; beyond this the electrification was negative, with a 

 potential of from 100 to 200 volts. There seemed to be a 

 neutral zone dividing the two opposite electrifications. 



Amber. — The amber was 3 cm in length ; one-half of it was 

 wrapt in tin-foil fastened down by thread. Being supported 

 by insulating silk thread, it was left for half an hour in con- 

 tact with the knob of a Leyden jar charged positively to 

 12,000 volts. On removal and discharge of the foil armature, 

 it was found charged positively, with a potential of 300 or 

 400 volts. 



Rosin. — A piece of freshly cut rosin of the same dimen- 

 sions and treated in the same way, became negatively electri- 

 fied. Other experiments given below tend to show, that 

 taking dry and damp weather together, plain rosin is the best 

 insulator that was examined. 



Insulating silk thread. — Ordinary thick white sewing silk 

 was passed through a bath of one part of gutta percha melted 



