90 



Mr. A. A. C. Swinton. 



cone, a much larger proportion of the ring was cut off in each case, 

 this being much more marked with a high vacuum when the 

 diameter of the ring was small, than with a low vacuum when the 

 diameter of the ring was large. 



Convergent and Divergent Cones produced by Magnetic Focussing. 



In order to discover whether the apparent hollowness of the con- 

 vergent and divergent cones of cathode rays as above observed, when 

 the focussing was performed by means of a spherical cathode, was in 

 any way due to the concave form of the cathode or to the fact that 

 the rays were converging or diverging, experiments were tried with 

 a tube having a flat aluminium cathode, the rays being caused to 

 converge to a focus by means of a powerful electro-magnet in the 

 manner described by the writer in his paper on " The effects of a 

 strong magnetic field upon electric discharges in vacuo." * 



The arrangement is shown in fig. 19, the carbon anti-cathode 

 screen, B, being movable, and not connected to the anode D, which 

 was contained in an annex to the tube. By increasing or decreasing 

 the power of an electro-magnet M, by moving it nearer to or further 

 away from the tube, and by moving the anti-cathode screen up and 

 down the tube, the cathode rays could be focussed on the anti-cathode 

 screen so as to form a circle of any desired size, the focus, which 

 appears to be exactly on the pole of the magnet, being, of course, 

 always beyond the anti-cathode. 



In order similarly to investigate a divergent cone of cathode rays 

 magnetically produced, a circular coil of wire, E, was employed 

 instead of the magnet in the manner recently described by Professor 

 Meming.f This coil, which had 72 turns of No. 18 S.W.G. size 

 wire, was supplied with 20 to 25 amperes of current from a storage 

 battery. It focussed the cathode rays at a point exactly central to 

 its own plane, from which they again diverged on to the anti-cathode 

 screen. 



With convergent and divergent cones of rays produced magnetically 

 in the above manner, there was no difficulty in showing that, under 

 suitable conditions, these cones acted as if they were hollow, giving 

 bright rings of varying sizes, sometimes with and sometimes without 

 bright central spots, upon the carbon anti-cathode screen, exactly 

 similar in appearance to those obtained with the concave cathode. 



Further observations were as follows : — ■ 



In some instances, two concentric hollow rings were observed, 

 especially with a low vacuum, when the magnet was suddenly turned 



* ' Koy. Soc. Proc.,' 1896, yoI. 60, p. 179. 

 f ' Electrician,' January 1, 1897. 



