PART ii. POLAR MAGNETIC: PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. vi. 



6?5 



As soon as these shells were put on outside the silvered globe, I obtained point-discharges in great 

 umbers; but they were not so intense as I had expected, not even when a large condenser was placed 

 i parallel with the vacuum-tube. It was only after having exhausted my discharge-box for a long time 

 nd filled it with hydrogen, and again and again exhausted it, that these point-discharges began to be 

 owerful. 



Figs. 260 a, b, and c show three photographs of discharges under varied conditions. 



The first is of an experiment with a considerable gas-pressure and very slight magnetisation of the 

 lobe. It shows an interesting radiation from the polar regions, but the point-discharges, which, it is 

 uc, are most numerous in the equatorial regions, have not separated into two zones as they usually 

 Id when the surface of the cathode was smooth. 



The third photograph is of an experiment in hydrogen gas with a very high vacuum. 



The phenomena here were powerful and sometimes of a distinct duration, that is to say not 

 istantaneous discharges. Another interesting circumstance is that under the above-mentioned experimental 



' 



a be 



Fig. 260 



nnditions it was distinctly seen that the patches are not always single spots, but often consist of a group 



spots. For instance, on the original photograph answering to Fig. 260 c, the spot above to the right 



distinctly a group of 5 separate spots. We thus have here another analogy to the sun-spot conditions. 



As I have frequently mentioned, I have tried in vain to photograph the vortices that sometimes 



i-velope into great beauty round the points of light in these point-discharges on the globe-cathode. I 



ive said that the motion of these vortices is always counter-clockwise on the upper hemisphere, and 



ockwise on the lower, supposing the globe to have been magnetised so as to have a magnetic north 



,'lc in the upper hemisphere. In the reverse case, the conditions are of course reversed. 



A chance occurrence has now enabled me to produce these vortices with much greater brilliancy 

 i an before. It was as follows. The vacuum-box was exhausted by a rotary mercury-pump (Gaede 

 :mp), with a rotary oil-pump in series, both pumps being worked by small electric motors that were 

 innected with the electric current system of the town. Sometimes, in cases of necessity, I left the pumps 

 orking while we were absent from the Institute. 



On one occasion the tension was broken off, so that the motor stopped; and notwithstanding my 

 : lf-closing valves, the vaseline-oil from the oil-pump passed through the mercury-pump and into my 

 irge vacuum-box. 



