Rays of Positive Electricity from the Wehnelt Cathode. 927 



lime cathode 0, back of which is placed the accelerating 

 condenser Ac. In line with these are placed the nearly 

 coterminous magnetic and electrostatic fields. Beyond, and 

 fairly close, is the photographic plate P. The diameter of 

 the containing cylinder is about cm. With this arrange- 

 ment I got the first definite evidence of the positive carriers 

 on June 1st of the present year, and photographs that 

 admitted of exact measurement followed a few days later. 

 ]t was soon observed that with high accelerating fields 

 sharp photographs resulted only when the plate P is but 

 a few cms. from the emerging beam. At 3 cm. the photo- 

 graphs are blurred and indistinct. To date, 26 photographic 

 plates have been exposed under varying conditions of 

 discharge, magnetic and electric fields, accelerating field, 

 and distance of plate from the emerging beam. On many 

 of the plates more than one exposure was made. Of the 

 above number I have only 7 plates on which the lines are 

 clearly enough cut to measure accurately. 



The vacua employed were the highest that could be 

 obtained by the aid of liquid air and charcoal. For these 

 vacua, in the apparatus as I have it constructed, 15 to IS 

 hundred volts are required to start the discharge even though 

 the platinum cathode be initially at a cherry-red heat. 

 However, when once started, the terminal pctential dif- 

 ference often dropped to one-quarter of the above value. 

 Slight fluctuations in the heating current produced corre- 

 spondingly large fluctuations in the discharge. This is due 

 to the lime cathode evolving gas very freely when heated. 

 In my apparatus the evolution of gas at white heat was 

 much more rapid than the two charcoal bulbs in liquid air 

 could absorb, in fact at times the charcoal seemed to refuse 

 to absorb it at all. 



The mao-netic field was measured by means of a Grassot 

 flux-meter and a triangular coil. The equation * that 

 applies is 



_ nd C l 



{l-x)Rdc 



where I is the magnetic induction through the coil as 

 measured by the flux-meter. 



For the electrostatic field I at first made use of the well- 

 known equation f 



*> = -^Xl(l+d) (1 



* J. J. Thomson, Phil. Mag-. Dec. 1909. 



t J. .*. Thomson, ' Conduction of Electricity through Gases/ 2nd ed. 

 p. 121 (1906j. 



