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Prof. J. J. Thomson. On the Discharge of [Feb. 13, 



III. £< On the Discharge of Electricity produced by the Rontgen 

 Rays, and the Effects produced by these Rays on Dielectrics 

 through which they pass." By J. J. Thomson, SLA., F.R.S., 

 Professor of Experimental Physics, Cambridge. Received 

 February 7, 1896. 



The Rontgen rays, when they fall upon electrified bodies, rapidly 

 discharge the electrification, whether this be positive or negative. 

 The arrangement I have used to investigate this effect is as follows : — 

 The Ruhmkorff coil and the exhausted bulb, used to produce the rays, 

 sire placed inside a large packing case covered with tin plate ; this is 

 done to screen off from the electrometer any electrostatic disturbance 

 due to the action of the coil. The needle of the electrometer is sus- 

 pended by a quartz fibre; thus, as there is no magnetic control, 

 the needle of the electrometer is not affected by changes in the 

 magnetisation of the core of the coil. 



The exhausted bulb is placed so that the phosphorescent part of it 

 is about 1^ inches from the top of the box, and a hole about an inch in 

 diameter is cut in the lid of the box just over the bulb, so as to allow 

 the rajs to emerge from the box ; a thin plate of either aluminium or 

 tin-foil is used to cover up the hole. The electrified plate, which 

 is a little larger than the hole, is placed outside the box about 

 2 inches above the hole in the lid, so that the Rontgen rays which 

 passed through the hole fall upon the plate. This plate is kept 

 permanently connected with one of the quadrants of a quadrant 

 electrometer ; the greatest care is taken with the insulation of this 

 j:>late and of the quadrants of the electrometer. The insulation was 

 so good that there was no appreciable leak when the coil was not in 

 action. The following is the method of making the experiments : 

 The two pairs of quadrants are connected together and the plate 

 charged to a high potential by an eleetrophorus or by temporary 

 connection with a large battery of small storage cells. All the quadrants 

 of the electrometer are now at the same potential. The two pairs of 

 quadrants are now disconnected; if the insulation is good the 

 potentials will remain the same, and there will be no deflection of the 

 electrometer; in our experiments the leak is so small that under 

 these circumstances the movement of the spot of light is hardly per- 

 ceptible. If, now, the Rontgen rays are directed on to the plate a 

 violent leakage of electricity from the plate occurs, the potential of 

 the quadrants connected with the plate changes, and in a few seconds 

 the spot of light reflected from the mirror of the electrometer is 

 driven off the scale. This leakage of electricity occurs whether the 

 plate is positively or negatively electrified ; if the plate is uncharged 



