90 



NATURE 



[May 26, 1898 



different velocities, on the well-known principle that occasions 

 the traffic in the street to form knots of maxima and minima, 

 owing to the faster vehicles catching up the slower, and being 

 impeded by them. 



Passing on to the production of X-rays in tubes of the 

 ordinary focus type, it is found that the particular material 

 employed for the anti-kathode surface considerably affects the 

 production of the Rontgen rays. This is a subject that was first 

 investigated by Prof. Sylvanus 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-kathode were 

 metals of the highest atomic weight. If, as seems probable, 

 the Rontgen rays are produced by the sudden removal of 

 velocity from the kathode ray atoms by collision with the anti- 

 kathode, this is in accordance with what would be expected, as 

 substances of high atomic weight would obviously be the most 

 efficient by reason of the greater inertia of their atoms. The 

 author has made numerous experiments with various metals for 

 the anti-kathode, comparing them in a tube in which the anti- 

 kathode, made half of one metal and half of another, was 

 movable. By jerking the tube, either half could be brought 

 opposite the kathode, and put into use ; so that under exactly 

 similar conditions it was possible to accurately compare the 

 efficiency of the two substances. Of available substances, 

 platinum was found to be much the best. 



The usual method adopted for varying the resistance of a 

 Rontgen ray tube, and thus modifying the character of the 



Fig. 2 



Rontgen rays it produces, so as to obtain the exact penetrative 

 quality that is desired, is by varying the vacuum. The higher 

 the exhaustion the greater is the resistance in the passage of the 

 discharge, the greater appears to be the velocity of the kathode 

 stream, and the more penetrative are the Rontgen 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, however, it is suggested that very possibly the temper- 

 ature of the contents of the tube and the consequent kinetic 

 energy of the molecules, which is greater the higher the 

 temperature, may in itself assist the passage of the discharge. 

 The author has found other means of varying the resistance of 

 the tube, and altering the character of the Rontgen rays that it 

 generates, which do not depend upon either the degree of 

 exhaustion or upon the temperature. According to one method 

 the tube is fitted with two or more kathodes of different sizes, 

 but all focussing upon the same anti-kathode. With such a 

 tube it is found that the smaller the kathode the greater is the 

 E.M.F. required to cause the electric discharge to pass through 

 the tube, and the more penetrative are the Rontgen rays 

 generated. Another method of effecting regulation consists in 

 making the anti-kathode, which is also the anode, movable, 

 and altering the distance between it and the kathode. Still 

 another, in making the kathode movable, and altering its 



NO. 1 49 I, VOL. 58] 



position relative to the glass walls of the tube. Some of the 

 author's experiments in these directions have already been 

 described in Nature for April 29 and May 27, 1897. He has, 

 however, now further studied the cause of these effects by means 

 of a tube in which the positions of both anode and kathode can 

 be altered independently by means of a magnetic adjustment. 

 Fig. 2 shows a portion of this tube, and above it is drawn a 

 curve representing, in terms of the alternative spark in air, 

 the difference of potential required to cause a discharge to pass 

 through the tube with varying positions of the anode. In the 

 diagram the abscissae represent the distance between anode 

 (which also formed the anti-kathode) and the kathode, divided 

 in tenths of an inch, while the ordinates represent also in tenths 

 of an inch the length of the alternative sparks in air betweerv 

 two brass balls i inch in diameter. Starting with the anode ir> 

 its furthest position from the kathode, and moving it gradually 

 towards the latter, it will be observed that at first there is a 

 slight gradual increase in the length of the alternative spark. 

 Then for the next small movement there is a very sudden in- 

 crease, and after that a further gradual increase till the point 

 marked in dotted lines is reached, which denotes the limit of 

 travel that the anode was allowed. Similarly, Fig. 3 represents 

 the effect of moving the kathode in the same tube, the anode 

 being stationary in the position shown. Here, as will be seen, 

 the less the distance between the kathode and anode the less is 

 the length of the alternative spark. This distance in this case 



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DISTANCE ; BETWEEN ELECTRODES 



Fig. 3. 



does not appear to be the determining factor, as it is more than 

 counterbalanced by the more important factor of the position 

 of the kathode relatively to the glass walls of the tube. Start- 

 ing with the kathode as far away as possible from the anode, 

 and moving it towards the latter, there is a gradual decrease in 

 the length of the alternative spark to commence with, then a 

 further, much more rapid decrease, as the kathode emerges from 

 the annex, and a still lurther, but less sudden decrease, as the 

 kathode is moved away from the glass walls out into the bulb. 

 Now as to the effect upon the Rontgen rays, as it has been 

 before remarked, the greater the resistance of the tube and the 

 greater the E.M.F. necessary to cause a discharge to pass, the 

 greater is the velocity of the atoms that form the kathode stream, 

 and the more penetrative are the Rontgen rays produced. 

 Further, so far as the movable kathode is concerned, the 

 supply of atoms appears to be of great importance. If penetra- 

 tive Rontgen rays are desired, the access of atoms to the kathode 

 must be restricted. If only a few atoms can get to the kathode, 

 these are projected at great velocity ; if there is too ready access, 

 the atoms crowd in upon the kathode, and the electrical charge 

 of the latter is unable to throw them off with much speed. It 

 is possible to restrict the supply of atoms to the kathode either 

 by bringing the latter back into a recess or annex, as in the tube 

 just shown, or by using a tube in which both kathode and anti- 



