betiveen the Terminals of a Vacuum Tube. 159 



As the potential of the tube evidently depended upon the 

 position occupied by the disc some experiments were made to 

 find out at what part of the tube the disc exerted the maximum 

 effect. With this object a tube was constructed about 30 cm. in 

 length, and having an iron disc which could be moved into any 

 required position by means of a magnet and the spark potential 

 found by the micrometer for the various positions. Starting with 

 the disc in contact with the anode and moving it towards the 

 cathode, the potential rose slightly at first, then fell a little, and 

 remained constant until the disc neared the cathode. The potential 

 then increased, reached a maximum and fell off a little when in 

 contact with the cathode. 



The rise in potential began when the disc entered the cathode 

 dark space, and the position of the maximum depended upon the 

 degree of exhaustion. 



The following is a typical example : 



Spark length 

 03 cm. 

 0-5 cm. 

 0-9 cm. 

 1 *2 cm. 

 1*4 cm. 

 1 - 4 cm. 

 1-2 cm. 



The length of the dark space was about 2 - 5 cm. but it was 

 difficult to locate the exact termination owing to the small diameter 

 of the tube and the bright phosphorescence of the glass. See 

 fig. 3 and curve (1). 



As it seemed probable that the increase in potential was due to 

 the cathode rays being drawn together by the disc, a tube was 

 made in which the cathode was an aluminium wire 1 mm. in 

 diameter, and covered with glass except at the extremity so that 

 the discharge proceeded from practically a point, fig. o, A, B, C. 



With this tube, using an iron disc, the following results were 

 obtained (curve 2) : 



Distance of disc from cathode Spark length 



cm. 0*4 cm. 



0-7 cm. 0-8 cm. 



10 cm. 0*6 cm. 



1-2 cm. 0-35 cm. 



2 - cm. and beyond 0*25 cm. 



1*5 cm. 0'33cm. 



1*0 cm. 0*55 cm. 



0-8 cm. 0-7 cm. 



11—2 



