686 



Mr. Alan A. Campbell Swinton 



[Feb. 4, 



Fig. 5 is a radiometer tube, exactly similar in principle to those 

 of Crookes. It consists of an ordinary focus tube, on one side of 

 which a glass annex has been blown, containing a sliding carrier, 

 holding half inside a glass cup a small and delicately pivoted wheel 

 with mica vanes. By the employment of a magnet, which acts on a 

 piece of iron attached to the sliding carrier inside the tube, I can 

 move the wheel bodily, cither out into tlie centre of the tube, so that 

 the cathode stream impinges upjn the vanes, or back into the annex, 



Fig. 5. 



-Adjustable radiomtter tube for showing both cathode 

 and anode streams. 



when the vanes are quite outside of the cathode line of fire. When 

 the tube is put into operation in this latter position (that shown in 

 full lines in the illustration), immediately the current is turned on the 

 wheel begins slowly to revolve in a direction that indicates a stream 

 from the anode to the cathode. On the other hand, when the wheel 

 is moved out into the bulb (in the position indicated in dotted lines), 

 so that the cathode stream impinges upon the vanes, tlie wheel imme- 

 diately begins to revolve with great rapidity in the opposite dii ection. 



