592 3Ir. Alan A. Campbell Stvinton [Feb. 4, 



It is found that tlie smaller the cathode the greater is the E.M.F. 

 required to cause the electric discharge to pass through the tube, and 

 probably in consequence of this, and also perhaps because a less number 

 of atoms can get into tbe vicinity of the cathode at one time, the greater 

 is in all probability the velocity of the stream of atoms that form the 

 cathode rays. 



The particular material employed for the anti-cathode surface 

 also materially aifects the production of the Eoutgen rays. This is a 

 subject that was, I believe, first investigated by Professor Silvanus 

 Thompson, who found that the best absorbents were the best emitters 

 of the Rontgen rays — in other words, that the best materials for the 

 anti-cathode were metals of the higliest atomic weight. If the Eontgen 

 rays are produced by the sudden removal of velocity from the cathode 

 ray atoms, or by a sudden change in this velocity by collision with tlie 

 atoms of the anti-cathode, this is in accordance with what would be 

 expected, as substances of high atomic w' eight would obviously be the 

 most efiicient by reason of the greater inertia of tLeir atoms. 



I have made numercnis experiments with various metals for the 

 anti-cathode, and I have here a tube which has a movable anti-cathode 

 made half of aluminium and half of platinum. By jerking the tube, 

 -either the platinum or the aluminium i)ortions can be brought opposite 

 the cathode and put into use, so that under ex^^ctly similar conditions 

 as regards vacuum, size of cathode and bulb and distance, it is 

 possible accurately to compare the efficiency of the tw^o substances. 

 Fig. 12 is a j^hotograph of my wrist taken with the platinum portion 

 of the anti-cathode, and Fig. 13 one taken with the aluminium portion. 

 The conditions were otherwise identical, but, as is very obvious, the 

 result with the platinum is much superior to the other. 



The usual method adopted for varying the resistance of a Eontgen 

 ray tube, and thus modifying the character of the Eontgen rays that 

 it produces, so as to obtain the exact penetrative quality that is 

 desired, is by varying the vacuum. Tbe higher the exhaustion the 

 greater is the resistance to the passage of the discharge, the greater 

 is the velocity of the cathode rays, and the more penetrative are the 

 Eontgen rays. This variation of the vacuum is usually effected by 

 heating the tube, which has the effect of driving out into the interior 

 molecules of the residual gas condensed or occluded upon the glass. 

 Apart from this, very possibly the temperature of the contents of tlie 

 tube and the kinetic energy of the molecules, which is gieater the 

 higher the temperature, may in itself assist the passage of the dis- 

 charge. 



There are, however, other means of varying the resistance of a 

 tube and altering the character of the rays that it generat* s which do 

 not depend upon either the degree of exhaustion or upon the tempera- 

 ture. One method for effecting this regulation consi>t-! in making the 

 anti-cathode, which is also the anode, movable, and altering the dis- 

 tance between it and the cathode ; another in making the cathode 

 movable and altering its position relative to the glass walls of the tube. 



