1897.] Prof. Thomson, On the Cathode Rays. 243 



Monday, February 8, 1897. 



Mr F. Darwin, President, in the Chair. 



Dr A. C. Haddon, Christ's College, was elected a Fellow of the 

 Society. 



The following Communications were made : 



(1) On the Cathode Bays. By Professor J. J. Thomson. 



The experiments described in this paper are of two kinds : the 

 first set are on the electric charges carried along the rays, the 

 second on the deflection produced in these rays when they traverse 

 a uniform magnetic field. 



In the experiments on the electrical effects produced by the 

 rays, the cathode, a plane disc, was placed in a small side tube 

 fused on to a large bulb ; between this tube and the bulb there 

 was a thick earth-connected metal disc with a slit in it ; a pencil of 

 cathode rays shot through this slit into the bulb. In the bulb on 

 the side opposite to the slit there was an arrangement similar to 

 that used by Pen-in in his experiments on the charges carried by 

 the cathode rays : it consisted of two cylinders, one inside the 

 other ; the outer cylinder was connected with the earth and the 

 inner cylinder (which was insulated from the outer) was connected 

 with one pair of quadrants of an electrometer. Slits were cut in 

 the cylinder so that the cathode rays could pass through the slits 

 into the inside of the inner cylinder. The cylinders were placed 

 at a considerable distance from the direct line of the rays, so that 

 unless the rays were deflected by a magnet they did not enter the 

 cylinder. The charge in the cylinder produced by each make and 

 break of the coil was investigated. A slight charge was found 

 to pass into the cylinder even when it was not in the direct line 

 of the rays, due probably to a diffused charge sent out from the 

 tube through the slit into the bulb at each discharge of the coil ; 

 this charge was small ; it was generally negative but at high 

 exhaustions was frequently positive. When the rays were 

 deflected by a magnet so as to pass inside the cylinder the 

 cylinder received a strong negative charge ; the charge was large 

 as long as the phosphorescent patch was stopped by the cylinder, 

 small when by the motion of the magnet the patch was removed 

 to one side or another of the cylinder. This experiment seems 

 conclusively to show that there is a flow of negative electricity 



