the Residual Gaseous Matter in a Cvookes Tube. 389 



by the dotted lines, so that the cathode-stream impinges di- 

 rectly upon the vanes, or can be moved back to the position 

 shown in full lines in the illustration, when the vanes are 

 quite out of the cathode-stream. When the wheel is in the 

 former position the tube acts exactly as an ordinary Crookes 

 electric radiometer-tube, the wheel rotating one way or 

 another in the direction of the cathode discharge as the con- 

 cave cup or the flat-plate electrode is made cathode, the 

 rotation being much more rapid in the latter than in the 

 former case. 



On the other hand, if the wheel is moved well out of the 

 cathode-stream, provided the exhaustion is high enough, it is 

 found to rotate in the opposite direction to the cathode-rays, 

 that is to say, in the direction that indicates an atomic stream 

 from anode to cathode round the outside of the cathode-stream „ 

 The rotation will only take place with high exhaustion, and is 

 never so rapid as in the previous case when the vanes were 

 in the cathode-stream ; but it is faster and faster the higher 

 the exhaustion, and at very high vacua the speed is very con- 

 siderable. The wheel rotates whether the concave or the flat 

 plate is made cathode, but in either case in a direction oppo- 

 site to what it did when in the cathode-stream. 



Very little electric power is necessary to show this effect. 

 With a sufficiently high vacuum a small Wimshurst machine, 

 passing so little current through the tube that scarcely any 

 fluorescence is visible, will cause the wheel to rotate at a 

 speed of many turns per second ; and rotation can even be 

 produced by the mere approach of two oppositely charged 

 leyden-jars to the terminals of the tube. 



Further experiment appears to show that the rotation is 

 really due to a stream returning round about the cathode- 

 stream from anode to cathode, and that the atoms or particles 

 of which this stream consists are positively charged. In the 

 experiments so far described the anode was also the anti- 

 cathode ; and it is obvious that the stream might not be truly 

 an anode-stream, but merely a reflected or splashed cathode- 

 stream. It is, however, found that if the flat plate used as 

 anode be earthed, or if the plate be disconnected from the 

 electrical source of power, and used merely as an anti-cathode, 

 the spare electrode already alluded to opposite to the wheel 

 being employed as anode, the wheel refuses to rotate, though 

 rotation recommences immediately in the first case when the 

 earth-connexion is removed, and in the latter case when the 

 anode and anti-cathode are connected together. This appears 

 to be fairly conclusive evidence that it is a stream projected 

 from the anode that causes the rotation of the wheel. It 



