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XXVI. Rays of Positive Electricity. 

 By Sir J. J. Thomson *. 



[Plate I.] 



IN the 'Philosophical Magazine' for October 1910 I 

 described some experiments on positive rays which were 

 made with very large discharge-tubes ; with such tubes it is 

 possible to work at very low pressures without the difference 

 of potential between the electrodes increasing to such an 

 extent as to endanger the tube by sparking through it. At 

 these low pressures I found that the patterns on the phos- 

 phorescent screen produced by the positive rays after they had 

 passed through electric and magnetic fields, were separate para- 

 bolic curves ; the value of ejm for the particles striking the 

 curve along one of these parabolas is constant. Parabolas 

 giving values of ejm corresponding to the hydrogen atom, 

 the hydrogen molecule, the helium atom, the carbon atom, 

 the oxygen atom, and the mercury atom were observed. I 

 had previously, in 1907 (Phil. Mag. xiii. p. 561), observed 

 the curves corresponding to the hydrogen atom and molecule, 

 and also to the helium atom. Jn all these experiments the 

 rays were detected by the phosphorescence they produced on 

 a willemite screen ; such a screen, however, does not give a 

 permanent record of the curves traced on it by the positive 

 rays, and accurate measurements of the dimensions and 

 positions of the luminous curves are more difficult than they 

 would be on a photograph. For these reasons I have en- 

 deavoured to apply photographic methods to the study of 

 these rays : at first 1 attempted to photograph the luminosity 

 on the screen, using a very large portrait-lens ; I abandoned 

 this method because I found that to get any effect on the 

 photographic plate exposures lasting several hours were 

 required. Apart from the tediousness of this process, it is 

 difficult to keep the conditions in the tube sufficiently constant 

 for so long a time. I see, however, that Dechend and 

 Hammer (Sitz. Held. Ah. 1910) have recently succeeded in 

 photographing the curves corresponding to several gases in 

 this way. 



I find that a much more sensitive and expeditious method 

 is to put a photographic plate inside the tube itself and let 

 the positive rays fall directly on the plate instead of on to the 

 willemite screen. The photographic plate is very sensitive 

 to these rays, and the places where they fall are recorded 

 when the plate is developed. The plate is much more sensitive 



* Communicated by the Author. 

 Phil. Mag. S. G. Vol. 21. No. 122. Feb. 1911. Q 



