204 Rood — Experiments on High Electrical Resistance. 



which had been damp. Wishing to improve the resisting 

 power of ebonite against moisture, I painted a similar piece 

 with boiled linseed oil, and allowed it to dry for two weeks. 

 It was not improved as shown by my electrometer, and to 

 avoid all uncertainty, I requested Mr. EL C. Parker to measure 

 its resistance by the " direct deflection method " with a galva- 

 nometer ; it turned out to be only 5300 megohms. It was 

 then gently heated by a spirit lamp and while warm measured ; 

 the resistance had been raised to 90,000 megohms ; it fell at 

 first quite rapidly, and afterwards slowly, finally reaching, in 

 four minutes, 6000 megohms. It was then heated to a tem- 

 perature of 100° C. for four hours, and after free exposure to 

 the air for three days, it was found that its resistance had been 

 increased fifteen fold, the hygrometer standing at 54° when it 

 was measured. 



One more experiment may be given ; on a somewhat damp 

 day the hygrometer stood at 56°, the thermometer at 80° F. ; 

 pieces of mica and thin ground glass were prepared with 

 metallic armatures; the pieces were 13 mm broad and the 

 distance from armature to armature was 55 mm . The resist- 

 ance of the glass was found to be nearly one thousand megohms, 

 that of the mica 50,000 megohms. 



In the preliminary measurements that have been detailed in 

 this article, it has been assumed that during the transit of the 

 electricity there is practically no loss by convection nor by 

 leakage in the electrometer, so that the time consumed by a 

 given charge in traversing a conductor and charging the elec- 

 trometer to a given potential, would be directly proportional to 

 the resistance of the conductor, and the few experiments that I 

 have been able to make seem to justify this conclusion, but 

 my set of high resistances was not completed in time to give 

 it a rigorous examination. In dry weather the Leyden jar lost 

 only two-thirds of its charge in twelve hours, although its 

 necessary construction was far less favorable for insulation 

 than was the case with all of the other arrangements. 



The measurements given in this article were made with the 

 help of certain standard units that I have recently constructed 

 on a new plan. A considerable number were made, the resist- 

 ance in each case not exceeding 5000 megohms. These were 

 measured separately by Mr. H. C. Parker with a galvanometer 

 and the "direct deflection method." Afterwards, they were 

 joined together, and furnished suitable standards. 



It may finally be remarked that almost all of the experi- 

 ments described in this article are quite easy of execution, and 

 many of them are well adapted for class illustrations. 



June 18th, 1900. 



