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XXXVIII. On the ( i rctilatiou of the Residual Gaseous Matter 

 in a Crookes Tube. By Alan A. Campbell Swinton*. 



TI^HEKE appears to be generally some doubt as to the way 

 JL in which the particles or atoms that form the cathode- 

 rays in a Crookes tube return again to the cathode. It is 

 obvious that they must return, as otherwise they would all 

 become collected at one end of the tube, and the cathode 

 would soon be surrounded by an absolute vacuum . By some 

 it has been supposed that the atoms return by ordinary dif- 

 fusion in the intervals between the succeeding electrical 

 discharges ; by some that they creep back along the inner 

 surface of the glass walls of the tube ; by others that they 

 return during a discharge through the space between the 

 cathode-rays and the glass. Further, there is the question 

 whether the returning atoms carry any portion of the positive 

 electricity from the anode to the cathode, similarly as part at 

 any rate of the negative electricity is believed to be carried 

 from the cathode to the anodic portion of the tube by the 

 cathode-ray atoms. 



In his 1891 Presidential Address to the Institution of 

 Electrical Engineers, Sir William Crookes described a tube 

 which was divided into two halves by a diaphragm pierced 

 with two small apertures Near each aperture was mounted 

 a small wheel with vanes to detect and show the direction of 

 auy stream of atoms that might pass through. It was found 

 that when the cathode was caused to project rays through one 

 aperture, the rotation of the wheel at the other aperture showed 

 the atoms in the act of returning. 



In this tube, however, both anode and cathode were on the 

 same side of the diaphragm, the anode being behind the 

 cathode. So the experiment cannot be said to decide the 

 existence of a true anode-stream, but merely to demonstrate 

 that the action of the cathode-stream was to create a differ- 

 ence of gaseous pressure in the two halves of the tube which 

 relieved itself by a current of atoms through the spare aper- 

 ture in the diaphragm. 



The writer, with the assistance of Mr. J. C. M. Stanton 

 and Mr. H. Tyson Wolff, has investigated the matter further 

 by means of a series of tubes, one of which is illustrated in 

 fig. 1. In this tube, which is very highly exhausted, we 

 have as two electrodes a concave aluminium cup and an 

 aluminium plate placed opposite to one another, as in an 

 ordinary focus-tube. As will be seen, there is also a supple- 

 mental wire electrode at one side. Inside the tube is a very 



* Communicated by the Physical Society : read Mar. 25, 1898. 



