174 Mr. C. E. S. Phillips. Action of Magnetised Electrodes 



" made," and it was more sharply defined at high exhaustions : becom- 

 ing in fact hazy and indefinite, if the pressure within the bulb was 

 slightly increased. 



On the other hand the rarefaction must not be carried too far, for it 

 is necessary, in order to obtain this luminous effect, that the residual 

 gas within the bulb should be very generally stimulated by the passage 

 of the discharge. The following combinations were then tried : 



After stimulation, both the leading wires attached to the electrodes 

 were removed from the secondary of the induction coil, and (a) insu- 

 lated, (b) joined together and insulated, (c) joined together and 

 connected to earth, (d) one insulated and the other connected to earth. 

 In all these cases the ring formed equally well when the pointed ends 

 of the electrodes were oppositely magnetised. On the other hand, as 

 long as the points were made either both N or both S, no ring could 

 be obtained. 



In another experiment, after the exhaustion had been carried some- 

 what further and the bulb strongly stimulated, a second ring flashed 

 out momentarily when the magnet circuit was completed ; it formed 

 concentrically with the smaller and more permanent ring, and appeared 

 to be situated upon the inner surface of the glass bulb. 



Observations as to the actual mode of formation of what I venture 

 to call the primary ring, i.e., the smaller one of the two, could at this 

 stage of exhaustion be conveniently made. It appeared to emanate 

 originally as a bright stream from between the electrodes, and then 

 to curl rapidly round the magnetic axis, that portion most distant 

 from the electrodes gaining upon the rest, ultimately disengaging the 

 tail of the stream from between the points, and thus forming an equa- 

 torial circle of light within the bulb. The ring then spread out and 

 became somewhat wider and less well defined, and as it gradually died 

 away the glow seemed to be rotating more and more slowly until at 

 last it flickered and vanished. 



It appears, in fact, that this luminous ring spins between the electrodes 

 from the moment it forms under the action of the magnet, the high 

 initial velocity with which, in that case, it must be set in motion tending 

 to keep it rotating, even after the magnetic lines have reached a maxi- 

 mum. The gradual expansion of the ring, which begins to take place 

 immediately it has formed, may, according to this view, be due partly to 

 centrifugal force, and also partly to the attraction exerted by an electro- 

 static charge residing upon the inner surface of the glass walls of the 

 bulb. It is significant, too, that when the ring had all but disappeared, 

 the sudden turning off of the magnet slightly revived the luminosity. 

 At the instant the ring formed, the glass walls of the bulb became 

 charged so strongly that a spark could, in some cases, be seen to pass 

 between the outer surface of the glass and the brass cups, C 15 C 2 , attached 

 respectively to either electrode. It should be noticed that the ring is 



