Rays of Positive Electricity. 42 7 



unaffected by changes in the pressure. For assuming that 

 the velocity of the positive rays in this boundary does not 

 remain constant, then if y is the deflexion due to the magnetic 

 force H, x that due to the electric force X, then y is pro- 

 portional to He/mv and x to ILe/mv 2 : thus for a particle 

 on the unaltered boundary e/mv must be constant as the 

 pressure changes, and X must be proportional to v : it would, 

 thus require a very elaborate adjustment of variable quan- 

 tities to keep the boundary fixed. I thought it, however,, 

 desirable to test this point by direct experiment, and tried 

 several methods of doing so. In one way I used a tube in 

 which the only connexion between the part of the tube where 

 the discharge took place and the part traversed by the rajs 

 after passing through the cathode, was through the long, 

 hollow, narrow hypodermic needle through which the rays 

 passed. With this arrangement, it took some time for the 

 pressures in the two tubes to get equalized if a difference 

 between them was produced in any way. Thus it was- 

 possible to have for a short time the pressure considerably 

 higher in the discharge-tube than in the region traversed 

 by the rays after passing through the cathode. The 

 phosphorescence was first observed when both the discharge- 

 tube and the space through which the rays passed was ex- 

 hausted to a very low pressure; a little air was then suddenly 

 let into the discharge part of the tube, so that for a short 

 time the positive rays were produced in a gas at compara- 

 tively high pressure and then passed through a gas at very 

 much lower pressure on their way to the willemite screen- 

 In this case there was no displacement of the left-hand 

 boundary of the phosphorescent patch, though now the 

 electric force acting on the charged particles must have been 

 the same as it was before the pressure in the discharge-tube 

 was increased. 



Another method I tried was to reverse i he direction of the 

 discharge, and use the perforated electrode as the anode: in 

 this case a stream of cathode rays passed through the needle 

 and fell on the willemite plate. The deflexion of these rays 

 was measured under the same electric field with different 

 pressures in the discharge-tube; and it was found that the 

 deflexion of these cathode rays increased enormously as the 

 pressure in the discharge-tube was increased. It was quite 

 easy to get at the higher pressures deflexions ten times as- 

 great as those at lower ones; the magnitude of the deflexions 

 varied approximately inversely as the potential-difference 

 between the electrodes, which is the result we should expect 



