578 Professor J. J. Thomson [March 22, 



summary of the properties of these rays is contained in a paper by 

 Ewers.* 



As these rays seem the most promising subjects for investigating 

 the nature of positive electricity, I have made a series of determina- 

 tions of the vahies of ^ for positive rays under different conditions. 

 m 



The results of these I will now proceed to describe. 



Apparatus. 



Screen used to Detect the Rays. — The rays were detected and their 

 position determined by the phosphorescence they produced on a screen 

 at the end of the discharge-tube. A considerable number of sub- 

 stances were examined to find the one which would fluoresce most 

 brightly under the action of the rays. As the result of these trials 

 willemite was selected. This was ground to a very fine powder and 

 dusted uniformly over a flat plate of glass. Considerable trouble 

 was found in obtaining a suitable substance to make the powder 

 adhere to the glass. All gums, etc., when bombarded by the rays 

 are liable to give off gas ; this renders them useless for work in 

 vacuum-tubes. The method flnally adopted was to smear a thin 

 layer of " water-glass " (sodium-silicate) over the glass plate, and then 

 dust the powdered willemite over this layer and allow the water- 

 glass to dry slowly before fastening the plate to the end of the tube. 



The form of tube adopted is shown in Fig. 2. A hole is bored 

 through the cathode, and this hole leads to a very fine tube F. The 

 bore of this tube is made as fine as possible, so as to get a small well- 

 defined fluorescent patch on the screen. These tubes were either 

 carefully-made glass tubes, or else the hollow thin needles used for 

 hypodermic injections, which I find answer excellently for this pur- 

 pose. After getting through the needle, the positive rays on their 

 way down the tube pass between two parallel aluminium plates A A. 

 These plates are vertical, so that when they are maintained at different 

 potentials the rays are subject to a horizontal electric force, which 

 produces a horizontal deflection of the patch of light on the screen. 

 The part of the tube containing the parallel aluminium plates is 

 narrow^ed as much as possible, and passes between the poles P P of a 

 powerful electromagnet of the Du Bois type. The poles of this 

 magnet are as close together as the glass tube will permit, and are 

 arranged so that the lines of magnetic force are horizontal and at 

 right angles to the path of the rays. The magnetic force produces a 

 vertical deflection of the patch of phosphorescence on the screen. 

 To bend the positive rays it is necessary to use strong magnetic fields, 

 and if any of the lines of force were to stray into the discharge-tube 



* Jahrbuch der Radioaktivifat, iii. p. 291 (1906). 



