Insulating Properties of Indiarubber. 505 



a readable current was connected to the voltmeter, and at a 

 given time the circuit containing the condenser and the 

 galvanometer was completed. The deflexion of the galva- 

 nometer was noted at the end of the first minute, and from 

 time to time afterwards. The current was found to decrease 

 rather rapidly at first, but when electrification had been con- 

 tinued for about an hour it became approximately constant, 

 especially in the case of the pure rubber condensers. The 

 pressure was then raised and electrification continued until 

 the current again became steady, when the pressure was 

 again raised. This was continued for five or six different 

 pressures, and the values of resistance when the current had 

 become steady were plotted as ordinates, and the correspond- 

 ing voltage as abscissae, giving a curve of resistance with 

 pressure. In the case of the okonite cable an experiment 

 was also made in which a series of readings of the current 

 were taken with the pressure increasing in steps to a maximum, 

 and then decreasing in the same steps. 



7. In the experiments to determine the form of the curves 

 of charge and discharge from the cable, it was found possible 

 to take readings for three different periods of electrification 

 in one day, the copper conductor being kept " earthed " 

 between successive cbargings. In this manner a set of curves 

 of discharge after 5, 10, 40 minutes' charge was taken, the 

 temperature and charging pressure remaining practically 

 constant. The method of procedure was as follows: — The 

 cable having been short-circuited all night, the temperature 

 was determined by measuring the resistances of the cable and 

 the copper thermometer which floated in the tank. Then 

 the E.M.F. of the cells to be used was read on the electro- 

 static voltmeter, the leads being afterwards disconnected. 

 The cable was then charged for five minutes, and at the end 

 of that time, as shown on the chronometer, the discharge-key 

 was depressed, the galvanometer being short-circuited for the 

 first fifteen seconds of discharge. Steady readings could gene- 

 rally be obtained after 40 seconds; and these were continued 

 by noting on the chronometer the time at which the galva- 

 nometer spot passed successive divisions of the scale until the 

 deflexion had fallen to one centimetre. The galvanometer 

 was then standardized at different points over the whole 

 range used, and readings taken of the temperature and 

 battery volts. This was repeated at the end of two hours for 

 10 minutes' charge, and again at the end of two and a half 

 hours for 40 minutes' charge. On some days two discharges 

 only were taken, viz. : after 20 and 120 minutes' charge. By 

 this means it was found possible to get a set of discharge 



