PART. II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. IV. 



571 





Fig. 208. 



Of late, I have made these experiments partly by the employment of high-tensioned electric waves, 

 produced by DUDELL vibrations, through the discharge-tube. It seems to be a comparatively easy way 

 of producing them. I have, however, preferred a direct current from the previously-mentioned 2o,ooo-volt 

 machine (fig. 67), and have employed discharges of up to 35 milliamperes through the receptacle. 



The four photographs in fig. 208 were taken from experiments such as these. The terrella No. 4 

 employed was 8 cm. in diameter, and a current of from 10 to 12 amperes was employed upon it 

 (M = 28000 C. G. S.). The magnetic axis was set at an angle of about 30 with the axis of rotation, 

 and the magnetic equator was drawn in pencil upon the terrella, as we were to see whether the ring 

 coincided with the magnetic equator in all positions of the terrella. The first two photographs were taken 

 while the magnetic north pole (below) had an hour-angle of o. They were taken from places with hour- 

 angles of 90 and 270. 



It will be seen that although the angle between the magnetic axis and the axis of rotation is made 

 so great, the equatorial ring lies fairly parallel with the magnetic equator. The ring here is most 

 powerful!}' developed farthest from the cathode. 



