1896.] Electricity thro Dielectrics traversed by Rontgen Rays. 127 



reflected from the mirror of the quadrant electrometer, as soon 

 however as the rays passed through the dielectric there was 

 leakage through the dielectric, the potential of the disc approached 

 that of the aluminium plate and the spot of light moved across 

 the scale. The rate of leak was determined by measuring with a 

 stop-watch the time taken by the spot of light to pass across 

 50 scale divisions. The wire attached to the middle of the electro- 

 meter carried a vane which make the deflections of the electro- 

 meter dead beat. 



In the other method, which we shall call method (B), a mica 

 window was inserted in the side of a glass vessel shaped like a 

 lamp-glass, the window was placed opposite to the hole in the 

 tank containing the coil and bulb. The open ends of the glass 

 vessel were fitted with paraffin plugs, through each of these plugs a 

 metal rod passed carrying a metal disc, the two discs were parallel 

 and opposite the window. As long as the gas between the discs 

 insulated no electricity could pass from one disc to the other. 

 One of the discs was kept permanently connected with one 

 terminal of a battery of small storage cells, the other terminal of 

 which was connected with the earth, while the other disc was 

 connected to one pair of quadrants of the electrometer. The 

 other pair of quadrants were connected to earth. If the two pairs 

 of quadrants were connected together and then disconnected the 

 spot of light reflected from the mirror would remain at rest if the 

 gas between the discs (which are at different potentials) remained 

 an insulator, when however electricity can leak through the gas 

 the potentials of the two discs will tend towards equality and the 

 spot of light will move across the scale. The spot of light 

 remained at rest when the gas between the plate was not 

 traversed by the Rontgen rays ; as soon however as these passed 

 through it the spot of light began to move, and the conductivity 

 of the gas was determined by measuring the time taken for the 

 spot of light to pass over 50 scale divisions. 



Conductivity of different gases when traversed by the Rontgen 

 rays. The following results were determined by the method A. 

 The vessel was filled with air at atmospheric pressure and tem- 

 perature and the rate of leak when the rays were passing through 

 the air determined. The air was then pumped out and the 

 vessel filled with the gas to be examined and the rate of leak 

 again determined; after this the gas was again pumped out and 

 the vessel refilled with air and another determination of the rate 

 of leak made ; this process was repeated several times. 



For these determinations it is necessary to use a bulb whose 

 rate of emission of the Rontgen rays remains steady. There is 

 sometimes a little difficulty in getting the bulb into this state, 

 many bulbs which will produce satisfactory photographs are so 



