Rood — Experiments on High Electrical Resistance. 293 



with three of rosiD. Four centimeters of this was treated as 

 above and became feebly negatively electrified, with a poten- 

 tial of about 200 volts. The temperature of this bath is rather 

 high, and oue composed of wax and rosin probably would 

 answer all practical purposes. 



Is the conduction of electricity by glass entirely on the sur- 

 face, or does the electricity penetrate a little below the sur- 

 face? — Oue experiment bearing on this question is here given : 

 a sealed tube of soft German glass conducted quite well ; a 

 portion of it was heated till it melted black sealing wax, and 

 in this way a few centimeters of it were coated with the wax, 

 but when it was cold it conducted about the same as before. 

 An application of boiled linseed oil produced no effect on its 

 conducting power ; finally, a few centimeters of the surface 

 were ground away on a stone, but after washing, drying and 

 the application of heat, it conducted just as before. 



Experiments in damp weather. — The results of a few experi- 

 ments on the effects produced by dampness of the air on the 

 resisting power of certain insulators may be of interest, and 

 some are here given. 



Ebonite. — Hygrometer at 78° ; a piece of the same sample 

 previously mentioned was used ; the distance from armature 

 to armature was 17 mm ; the breadth 13 mm , thickness 2 mm ; it 

 had been exposed to the air for several days. Employing 110 

 volts at first, it conducted quite well, its resistance being about 

 5000 megohms. It was then warmed, and while cooling 

 showed a resistance as high as 1,200,000 megohms. In three 

 minutes this fell to 15,000 megohms and then remained con- 

 stant. Two hours later, the same figure was obtained. 



Gutta percha of the same dimensions with the ebonite, 

 without being warmed or dried in any way, had a resistance of 

 12,000 megohms. 



Rosin, with armatures of tin-foil, the distance between them 

 being only 9 mm , had a resistance of 1,500,000 megohms. It 

 had been exposed to the damp air for days, and was not 

 warmed or dried. These experiments were made on the same 

 day, but on the following day when the air was somewhat 

 drier, the hygrometer standing at 65°, the resistance of the 

 ebonite was 340,000 and that of the gutta percha 440,000 

 megohms. The resistance of the rosin was too great to be 

 measured without altering the distance of the plate P, which 

 at the moment was undesirable. Freshly cut natural beeswax 

 also appears to be a good insulator in damp weather. A few 

 days later, the hygrometer standing at 60°, the resistance of 

 the ebonite was again measured and turned out to be 1,100,000 

 megohms. It had in the meanwhile been exposed to the air 



