Fiff. ] , 



Rays of Positive Electricity. 425 



that if suitable precautions are taken the air-gap method may 

 be made to give very satisfactory results. 



Using the electrostatic machine I was able to get observa- 

 tions on the Canalstrahlen with potential-differences between 

 the electrodes varying from 40,000 to .'3000 volts; the voltage 

 depends upon the pressure in the discharge-tube, being much 

 greater when the pressure is very low than when it is con- 

 siderably higher. For this range of potential-differences and 

 pressures the maximum values of v the velocity and e/tn the 

 ratio of the electric charge to the mass remained unchanged. 

 As might be expected, the appearance of the phosphor- 

 escent patch produced on the Willemite screen by the 

 pencil or, Canalstrahlen, when these were deflected by electric 

 and magnetic forces in the way described in my former 

 papers, changes considerably as the pressure varies. 



At very low pressures the phosphorescent patch was very 

 bright and sharply defined. It consisted of a comparatively 

 faint straight portion OA (fig. 1), ending in a well-defined 

 spot A, joining on to this is a tail AB, in 

 shape approximately a parabola, touching the 

 vertical line of no electrostatic deflexion at 

 the position of the undeflected spot. In 

 many cases another bright spot could be 

 observed between and A. 



As the pressure of the gas in the tube 

 increased the phosphorescent patch became 

 more diffuse but the position of the left- 

 hand boundary remained unaltered, the spread of the 

 phosphorescent patch being to the right. This left-hand 

 boundary remained unaltered up to the highest pressures at 

 which the phosphorescence was visible; at this pressure the 

 potential-difference between the terminals of the discharge- 

 tube had fallen to 3000 volts. The appearance of the phos- 

 phorescent path at this pressure is represented in fig. '1. 



Since the left-hand boundary remains 

 unaltered there are among the Canalstrahlen 

 produced by a potential-difference of 3000 

 volts some particles moving a< fas! as the 

 fastest of those produced when the pressure 

 was very low and the potential-difference 

 40,000 volts, these particles too have the 

 same value of e/m at ali pressures. The 

 appearance of the phosphorescence at the 

 higher pressures shows that while the velocity 

 of the majority of the particles is less than the maximum 

 velocity, there are some which possess this maximum velocity 



Fig. 2. 



"I 

 B 



