562 Prof. J. J. Thomson on 



they are deflected by an electric force. By measuring 

 the deflexions under magnetic and electric forces, Wien 

 found by the usual methods the value of e/m and the velocity 

 of the rays. He found for the maximum value of e/m the 

 value of 10 4 , which is the same as that for an atom of hydrogen 

 in the electrolysis of solutions. A valuable summary of the 

 properties of these rays is contained in a paper by Ewers *. 



As these rays seem the most promising subjects for in- 

 vestigating the nature of positive electricity, I have made a 

 series of determinations of the values of e/m for positive rays 

 under different conditions. 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 substances 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 &c. when bombarded by the rays are 

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

 vacuum-tubes. The method finally 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 purpose. 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 deflexion of the 

 patch of light on the screen. The part of the tube con- 

 taining the parallel aluminium plates is narrowed 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 

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



