6 Cady — On the Energy of the Cathode Rays. 



It is to be noted that in Table III as also in Table I, the 

 measured value of iY/Q decreases with decreasing pressure 

 and cathode-current. 



In order to investigate this relation more closely, a series of 

 observations was made, varying pressure and cathode-current 

 within as wide limits as possible. The cathode-current could 

 be altered either by changing the number of accumulator 

 cells, or by varying the cadmium-iodide resistance. Owing to 

 the evolution of occluded gases from the cathode and its 

 gradual discoloration, causing sudden irregularities in potential, 

 it was impossible to get a perfectly consistent set of readings. 

 Still in general it was evident that the ratio i V/Q diminished 

 with decreasing i, at first rapidly, then more slowly. At high 

 pressures (>(>05 mm ), the ratio seemed to depend also upon the 

 pressure; but when the pressure had sunk below 0'03 mm , the 

 drift was hardly preceptible. 



These results led to the supposition that beside the cathode 

 rays, a fraction of the discharge current also was conducted 

 from the cathode to the thermo-element ; for as J. J. Thom- 

 son* has shown, the portion of gas transversed by the rays be- 

 comes a relatively good conductor, except at very low pres- 

 sures. This supposition was confirmed by inserting a battery 

 between c (fig. 1) and the earth, thereby charging thermo- 

 element and Faraday cylinder to a positive or negative poten- 

 tial n.\ This cannot affect the true value of the ratio i V/Q as 

 long as ^denotes the difference of potential between cathode 

 and thermo-element; but the supposed disturbing current 

 must grow stronger or weaker according as v is made positive 

 or negative. 



The following table shows how the apparent value of i V/Q 

 was correspondingly increased or diminished according to the 

 sign of v. 



Pressure. 



V 



V 



i'10 1 Amp. 



Q-10 5 g-cal. 



iV/Q. 



0-050 



+ 70 



4050 



3-54 



34-1 



1-01 



0-050 







3980 



2-64 



30-2 



0-84 



0-050 



-70 



3910 



2-29 



30-5 



0-71 



The potential of the cathode as measured on the electro- 

 meter is here V—v; as in all measurements, i denotes the 

 observed sum of cathode- and disturbing currents. 



The variations in the value of Q result from the fact that 

 the path of the rays was visibly altered while the thermo- 

 element was charged. 



* J. J. Thomson, Phil. Mag., xliv, p. 293, 1897. 



f The cylinder was here connected with the point c. Owing to incomplete 

 insulation between thermo-element and cylinder it was not practicable to connect 

 the latter directly to earth. 



