﻿406 Mr. Rollo Appleyard on Dielectrics. 



The small opaque white spots, observed within the celluloid 

 under the microscope, led to some further experiments to 

 investigate the effect of isolated foreign particles within the 

 substance and upon the surface of dielectrics. Coarse brass 

 filings were scattered as thickly as possible over one face of a 

 warmed strip of gutta-percha, 16 inches long, 2 inches wide, 

 and i inch thick. The insulation of such a strip is not exactly 

 what might be expected ; when tested even with 750 volts its 

 resistance was practically infinite, and was certainly greater 

 than 600,000 megohms. A charged gold-leaf electroscope 

 could not be discharged with such a strip in this condition. 

 If, however, a wet cloth was passed once over the r on- 

 metallic face of the strip, or if it was simply breathed upon, 

 the insulation fell to 5000 megohms, and the strip now easily 

 discharged the electroscope. A round rod of gutta-percha 

 can be warmed and rolled in a heap of brass filings so as to 

 appear almost like a brass rod, and such a rod does not 

 discharge the electroscope. This would be an instructive 

 experiment for schools and instrument-makers. 



"Sensitive " Dielectrics. 



The next experiments were made with rods formed of a 

 mixture of gutta-percha and brass filings melted together in 

 various proportions. The length of these rods was about 20 

 inches, and their diameter f of an inch. Contact was made 

 with the ends for about 1^ inches by tin-foil which was bound 

 round with wire. It is found that, for small proportions of 

 brass filings, the resistance between the ends of these rods is 

 exceedingly high, and this high resistance is maintained until 

 about 2 parts, by weight, of brass are mixed with one part, 

 by weight, of gutta-percha. Here a " critical " point of propor- 

 tionality between the two substances occurs, undei* which the 

 rods have a very low resistance, of something like one ohm ; 

 and above which the resistance is exceedingly high, and can 

 only be measured in megohms. Several rods were made at 

 or near this " critical " poirt, and in no case could a medium 

 resistance of, say, a few hundred ohms be attained. All the 

 rods were either of very high or very low resistance. 



The method of making them is to warm a sheet of gutta- 

 percha upon a hot plate, using French chalk to prevent 

 sticking. The filings are sprinkled in as soon as the sheet 

 becomes soft. The whole is then made up into a pudding, 

 which is again flattened out into a sheet ; this is repeated 

 until a good mixture is arrived at. The compound is then 

 rolled into rods. 



