No. 1 consisted of cut sheet 068 cm. thick. This was darker in appearance and rather less transparent than Para. It cut rather more easily than most pure rubbers and was less tenacious. On keeping it has become quite hard and almost brittle. No. 2 was pure Para rubber 051 cm. thick. It was very transparent and very tenacious, and has retained these properties even after keeping for two years. No. 3 was a vulcanized rubber, white on the surface and dark when cut. It was 093 centimetre thick. The cable tested was a 3/18 in., 2760 feet long, insulated to 208 in. diameter with vulcanized rubber, and having an outer covering of impregnated braid. The sheet indiarubber was made into condensers by fixing tinfoil on each side by means of indiarubber solution made with pure benzene. The tinfoil sheets were 8 in. x 6 in., the dielectric projecting along each edge an inch beyond the tinfoil in order to prevent leakage. Each condenser thus made was placed between two glass plates, a strip of tinfoil being brought out from each sheet to the upper side of the top glass plate in order to make connexion. The surface of the glass was coated with shellac to prevent leakage by condensation of moisture on the glass. The paraffined paper condenser was made of stout notepaper which was first dried in a hot oven and then dipped into melted paraffin-wax. Four sheets of this paper 10 in. x 8 in. were alternated with five sheets of tinfoil, and placed between two glass plates. The condenser was then put between two iron blocks heated to about 85° C., and thus the wax was heated and allowed to solidify and cool down under pressure. The mica used was very pure and transparent, and consisted of sheets 2 in. x 4 in. An endeavour was made to form the plates of the condenser by depositing a film of silver on the mica from a solution of silver nitrate and Rochelle salt. By this means, intimate contact would be made between the metal and the mica without the introduction of any other dielectric in the form of a fixing material, or the inclusion of films of air. This, however, was not successful, as out of 30 sheets silvered not one was obtained which would give steady readings when charged with a 100-volt battery. The plates of the condenser were therefore formed of sheets of tinfoil 1 in. x 3 in., which were alternated with the mica. sheets, and the tinfoil was kept in close contact with the mica by pressure alone. This was effected by holding the condenser between two brass plates which were bolted together by 5/16 in. bolts, the condenser being insulated from the frame by sheet ebonite. The mica condenser eventually used consisted of 13 sheets of mica varying from 0009 in. to 001 in. in thickness. 5. The experiments were carried out in the Pender Electrical Engineering Laboratory under the direction of Dr. J. A. Fleming. The measurements of current were made by means of a sensitive Kelvin Astatic Galvanometer of 6057 ohms resistance provided with a shunt-box. The E.M.F. of the battery of small Lithanode cells was measured during the earlier experiments by a Weston voltmeter, but as the current taken by this instrument was sufficient to cause the E.M.F. of the cells to fall during an experiment, a Kelvin electrostatic voltmeter was used instead. The battery, condenser, and galvanometer were connected up to a three-way key in such a way that, with the key in its normal position, the condenser was charged through the galvanometer, and when the key was depressed the plates of the condenser were connected directly to the galvanometer. The galvanometer was standardized by means of a Crompton potentiometer and standard resistances. In this way the current corresponding to various points on the scale over the whole range used in any particular experiment was determined by reference to the Clarke cell. The "okonite " cable was immersed in a tank of water 3 in. x 3 in. x 24 in. deep. The two ends of the copper conductor were brought out to a block of paraffin-wax having four mercury cups. To two of the latter the ends of the copper were joined; the other two mercury cups being connected to a Wheatstone-bridge in order that the resistance of the conductor might be measured during an experiment. The volume of water was so great that the variation in temperature was never more than 0°-2 C. during the day, while the resistance of the copper conductor never varied more than 0.2 per cent. in a day. The temperature of the dielectric was taken as a mean between that of the water and that of the copper conductor. To prevent leakage over the ends of the cable, the braid was removed to a length of 2 feet and the surface of the indiarubber kept clean. A Price's guardring was also used to test for end leakage. In the mica and paraffined-paper experiments no special precautions were taken to ensure constancy of temperature during an experiment, or to measure the actual temperature of the dielectric, as the experiments were only intended to ascertain whether equations of similar form to those found for the cable would satisfy the curves of charge and discharge for these dielectrics. 6. In the experiments to determine the variation of resistance with pressure, a battery of cells just sufficient to give 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 galvanometer 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 continued 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 corresponding voltage as abscissæ, 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 chargings. 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 electrostatic 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 generally be obtained after 40 seconds; and these were continued by noting on the chronometer the time at which the galvanometer 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 readings for 5, 10, 20, 40, and 120 minutes' charge, at 200 volts and a practically constant temperature. In these experiments readings were generally taken of the current during electrification; but separate experiments in which electrification was continued for longer periods were also made to ascertain the equations representing the charging current. Readings of the current both when charging and discharging the mica and the paraffined-paper condensers were also taken in a manner similar to that described above. 8. The curves in fig. 1 show the readings obtained by the method described in section 6 from a condenser made from the rubber called No. 1. Before this experiment was made, the condenser had been dried for some weeks in order to get rid of the benzene used in the solution by means of which the tinfoil was fixed to the rubber. From the curve of resistance with pressure given, it will be seen that this dielectric shows a considerable decrease in resistance as the pressure is increased. A large number of similar experiments, which have been made on this condenser and on three other condensers made from the same rubber, all give curves very similar to that shown in fig. 1. An endeavour was then made to obtain similar curves of resistance from the okonite cable. This cable was first put into the tank on June 1st, 1899, and was kept continuously in the water until the end of July. The results of the experiments made during this immersion are given in the Tables I. & II., and in fig. 2. In the first experiment on July 4th (see Table I.) readings of current were taken at five different voltages, increasing in steps from 52 to 249 volts. After 75 minutes' electrification at 52 volts, the resistance was increasing per cent. per minute. The pressure was then raised to 100 volts, and after 70 minutes' electrification this rate of increase amounted to 12 per cent. per minute. As the experiment proceeds the final reading at each pressure becomes more constant, until at the highest pressure the variation is only 24 per cent. per minute. It will be seen that the steady resistances at the first four pressures do not differ very much, but at the highest pressure, viz. 249 volts, the resistance shows a decrease of 12 per cent. On July 5th an endeavour was made to compare the curves of resistance with time at 51.1 volts and 1974 volts. The results are given in fig. 2. Two sets of readings at 51 volts were taken, one before and one after the experiment at 197 volts, the cable being shortcircuited for 12 hours between experiments. The mean of the two sets of readings at the lower voltage is shown for comparison as the broken line above the curve at 197 volts. It |