Rays of Positive Electricity. 753 



fairly uniform intensity ; there is also a fainter prolongation 

 AC of the band in the opposite direction to AB due to rays 

 which carry a negative charge. The velocities and the values 

 of e\m for the rays can be determined by measurements of 

 this band. For if y and x are the vertical and horizontal 

 deflexions of a ray striking the screen at P, then the velocity 

 of this ray is equal to C\y/x and the value of ejm to c 2 y 2 \x, 

 where c x and c 2 are constants depending on the strengths and 

 positions of the electric and magnetic fields. I have shown 

 (Phil. Mag. loc. cit.) that the velocity of the rays in this 

 case is practically independent of the potential difference 

 between the electrodes in the discharge-tube, and that we 

 could increase the potential difference from 3000 to 40,000 

 volts without appreciably increasing this velocity. With 

 small tubes the appearance I have just described is often the 

 only effect to be observed even when the pressure is reduced 

 close to the point at which it ceases to be possible to force 

 the discharge through the tube. 



When large discharge-tubes are used a much greater 

 variety of effects can be observed. I have used tubes with a 

 volume as large as 11 litres ; these, however, are somewhat 

 difficult to procure and not very convenient to work with. I 

 have found flasks having a volume of 2 litres, such as are used 

 for boiling-point determinations, large enough for most 

 purposes. 



A uniform and sensitive phosphorescent screen is of great 

 importance as there is often a considerable amount of detail 

 to be made out, and some of it too faint to be visible unless 

 the screen is a very good one. My assistant Mr. Everett 

 has lately succeeded in making very uniform screens by 

 grinding the willemite into exceedingly fine powder, then 

 shaking the powder up in alcohol and allowing it to settle 

 slowly from the alcohol on to a flat glass plate ; when the 

 deposit has reached the requisite thickness the rest of the sus- 

 pension is drawn off and the deposit allowed to dry ; when 

 dry it sticks quite firmly to the plate, and the deposit is much 

 more uniform than that obtained by the method I formerly 

 used of dusting powdered willemite on a glass plate smeared 

 with water glass. The screens soon lose their sensitiveness 

 if bombarded by the rays, and when any fine detail has to be 

 made out it is advisable to use a new screen or a part of the 

 screen not previously bombarded by the rays. 



The discharge-tube is shown in section in fig. 2 (p. 754). 

 The perforated cathode C protrudes well into the tube, the 

 rays pass through the hole in the cathode through the fine tube 



"Phil. Mag. S. 6. Vol. 20. No. 118. Oct. 1910. 3 D 



