ON THE ELECTRICAL RESISTANCE OF INSULATING MATERIALS. 69] 
Taste VII.— Wire covered with India-rubber and Gutta percha (No. 27). 
Pressure AR | 
é in atmo-| Resistance. 
Time spheres, R. T7AP. Remarks. 
iP: 
ea millions. ae x : 
2 0 0 77 . urrent had been on 12 hour. 
2 10 75 lh eres , 
218 150 100 0-0019 
2 27 225 111 0-0028 
2 40 300 127 
2 47 0 80 Immediately after pressure. 
2 55 0 75 Ten minutes after pressure. 
Tables I., IJ., and III. contain the results of experiments with gutta-percha 
cables. The gutta-percha-covered conductor used consisted of coils of copper 
strand (three wires), weighing 72 lbs. per knot, and covered to 0-26 inch with 
three coatings of gutta percha, with intervening thin layers of Chatterton’s 
compound. The length of each of the coils was about a knot, and two of 
them were always placed in the pressure-tank at the same time; but they 
were tested separately. 
The coils, which had been for twenty-four hours previously maintained at 
a constant temperature, were placed in the pressure-tank, which was kept 
during the experiments at the same temperature. One end was sealed, and 
the other passed through a stuffing-box in the side of the tank to the testing- 
board. 
When the coils were in the tank, a vacuum was first made, then the water 
admitted, and, finally, the pressure put on in stages of 75 atmospheres, until 
the pressure of 300 atmospheres was attained. 
The battery-power used was uniformly 200 elements (Daniell’s). 
The resistances given in the Tables are reduced from the deflections of a 
Dubois’ zine galvanometer, and are expressed in millions of mercury (métre) 
units. 
Table I. contains the results of some measurements of electrical resistance 
of the coils, noted after the current had been active for one and three minutes 
respectively, at various pressures. The cable was put to earth for a sufficient 
time between the readings, under different pressures, to prevent the residuum, 
or electrification by the current, of one test interfering with the values ob- 
tained for the next test. 
The columns headed —— in the Tables give the ratios of increase of re- 
sistance due toa unit of pressure. The other columns are self-explanatory. 
The experiments recorded in Table II. were made to determine the effect 
of pressure on the capacity for electrification of gutta percha. It is well 
known that the apparent insulation of gutta percha increases during the first 
half-hour from the moment the battery is put to it, after which it becomes 
nearly constant. The observations in this instance were continued for 19 
minutes after making contact with the battery. 
The first series of readings were made under atmospheric pressure after 1, 
4, 9,14, and 19 minutes’ continuance of current respectively. The cable was 
- then put, for two hours, to earth. Similar series were made under various 
pressures up to 300 atmospheres, putting the cable between each series to 
2x2 
