upon Electrical Discharge Phenomena in Rarefied Gases. 173 



screw thread cut upon it there came a helix of rays which gave rise to 

 the appearance of golden-green rings of fluorescence upon the inner 

 surface of the glass vessel B. When the electrodes were magnetised 

 these green rings twisted somewhat and moved forward, the direction 

 of rotation depending upon the polarity of the magnet emitting 

 the rays. 



It has already been noticed* that an external magnet, placed behind 

 the negative electrode will cause rotation of the green fluorescence 

 upon the walls of an exhausted tube, and that the direction of motion 

 is opposite to what it would be in the case of a wire placed at right 

 angles to the axis of the magnet, and in which a current flowed away 

 from the pole. In the above experiment, the rotation was only partial 

 but agreed in direction with the results of other observers. 



The bending forward of the rays is evidently dependent upon their 

 partial rotation, because, on reversing the polarity of the magnet 

 emitting them, the direction of their rotation was also reversed, and the 

 bending forward still occurred. That a positively charged body, mov- 

 ing in a definite direction, sets up magnetic whirls in planes transverse 

 to its path, is generally accepted, the whole effect indeed being treated 

 as a current flowing in the same direction ; and it seems only logical to 

 conclude that were the body negatively charged, its motion would give 

 rise to effects similar to those accompanying a flow of current in the 

 opposite direction. From this we should expect cathode rays to behave 

 towards a magnet just as would a wire carrying a current in the 

 opposite direction to that in which the charged particles, constituting 

 the rays, are supposed to be moving a view which is borne out by 

 experiment. 



The pressure was then still further lowered, until a 3-inch spark 

 from a 10-inch Apps induction coil was only just able to start the 

 glow. Under these conditions irregular green patches flickered upon 

 the inner surface of the glass ; but when the electrodes were oppositely 

 magnetised these green flecks immediately vanished, and, the resistance 

 of the residual gas within the bulb becoming smaller, a hazy blue 

 cloud formed between the points. 



Owing to the ever varying charges upon the inner surface of the 

 bulb and upon the electrodes themselves, it could not be ascertained 

 whether this blue cloud tended to assume a definite geometrical form or 

 not. It was found, however, that, after a strong stimulation of the bulb 

 had taken place and then been stopped, the electrodes meanwhile 

 remaining unmagnetised, on exciting the magnet, a luminous ring 

 suddenly appeared within the bulb, between the pointed ends of the 

 electrodes, and in a plane at right angles to the direction of the mag- 

 netic lines of force. 



The ring shone brightly for a moment, when the magnet circuit was 



* ' Phil. Trans,' 1879, Part II, p. 657. 



