ON THE ELECTRICAL RESISTANCE OF INSULATING MATERIALS. 693 
case of gutta percha, the increase of electrification is proportional to the in- 
crease of electrical resistance by pressure, the electrification of india-rubber 
decreases with the pressure in a ratio surpassing the decrease of electrical 
resistance. 
Struck by this extraordinary difference in the effects of pressure upon these 
two materials, I next submitted to my tests a wire covered first with india- 
rubber by the longitudinal process, and thereupon with gutta percha to the 
thickness of 0°175 inch. 
In submitting a wire so covered to pressure, I made certain that the results 
obtained could not be influenced by any direct action of the water upon the 
india-rubber by absorption or otherwise, owing to the intervention of the 
gutta percha. 
Tables VI. and VII. contain the resistances of the insulating materials at 
different pressures, the electrification being completed. 
These observations are interesting in so far as they confirm the foregoing 
results with gutta percha and india-rubber, the coefficients being a mean 
between those of the two materials separately, as will be seen by the 
following :— 
Mean of coefficients for gutta percha from Tables I., IT., and III... + 0-0041. 
The same for india-rubber from Tables IV. and V. ............ —0-0009. 
The same for both these materials combined from Tables VI. and VII. +0-0016. 
The accompanying diagram, constructed from Tables III., IV., and VILI., 
is added to give a graphic representation of the variations observed in the 
f Gutta percha. 
and 
India-rubber. 
Resistance. 
— 
See India-rubber. 
| 
sm 
100 200 300 atmospheres. 
Pressure. 
electrical resistances under increasing pressures. The abscissee represent 
pressures, and the ordinates the electrical resistances when the maximum of 
electrification was attained. 
