PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. IV. 577 



It now, however, appeared to be impossible to obtain, by means of rays through the slit, any 

 precipitation on the terrella at a place answering to 6 />. in., even if the magnetisation of the terrella 

 were altered, as long as the magnetic axis coincided with the axis of rotation. 



It is possible that if the discharge-box had been much larger, returning rays of this kind might have 

 been made, by high magnetisation, to descend upon the terrella in places answering to 6 p. m. This 

 question will be taken up again for thorough investigation, later on. In the mean time, experiments 

 were made in letting the magnetic axis of the terrella form an angle of about 20 with the axis of 

 rotation, once so that the south pole turned towards the cathode, and another time so that it turned 

 away from the cathode. This latter position must answer more or less to the condition of the magnetic 

 axis upon the earth in winter. Experiment showed that if the magnetic south pole were turned towards 

 the cathode, the precipitation from the rays through the slit was nearest the terrella in places answering 

 rather to earlier hours than 3 p. m. than to later. 



On the other hand, the experiments showed decidedly that when the magnetic south pole was 

 turned away from the cathode, an abundant precipitation fell upon the terrella in places answering to 

 6 p. m. 



Nos. 4, 5 and 6 were from an experiment in which the pressure was 0.0012 mm., the discharge- 

 current 21 milliamperes, the tension 2100 volts, and the magnetic current to the terrella 7 amperes. 

 They were taken from places with hour-angles of 90, 180 and 330. 



The screen, with the slit and the hole in it, had a position answering to an hour-angle of 80. 

 The magnetic axis formed an angle of 20 with the axis of rotation, and the south pole was in the 

 position of a place having an hour-angle of 180. No. 5 shows how the rays through the slit and the 

 hole have turned back and strike the screen. 



We have seen that in all the numerous experiments mentioned here, the rays divide into two 

 groups, which we have called A and B. The first group comprises rays whose course is about the 

 equatorial plane, and which turn alternately up and down, above and below that plane, twisting about 

 the terrella in a direction from west to east. The boundaries of the group upon the terrella are formed 

 of those rays which turn so far out from the equator that they form polar precipitation. We have 

 assumed that corresponding precipitation upon the earth forms what we have called the negative polar 

 storms. 



The second group of rays approaches the terrella in the north and south polar regions, and the 

 rays descend in the polar belt with a velocity-component tangential to the terrella in a direction 

 opposite to that of the rays of group A. 



We may therefore assume that rays of this kind on the earth glance off into the auroral zone 

 with a movement from east to west, and thus occasion what we have called positive polar storms. 



That the rays about the equator must curve in the reverse way to those over the polar regions 

 of the terrella, is a consequence of the fact that the magnetic lines of force run in opposite directions 

 in the two places. 



We will now go on to the further experiments that were made for the purpose of studying the 

 polar rays. 



Photograph 7 was taken from a place with an hour-angle of 180, with the screen at 85. It 

 shows a bright precipitation of rays that have returned after passing through the slit and the hole. 

 The pressure during the experiment was 0.0014 mm., the discharge-current 21 milliamperes, the tension 

 3000 volts, and the magnetising current 7 amperes. 



Nos. 8 and 9 were taken under similar conditions, except that the position of the screen had an 

 hour-angle of 90, and the photographs were taken from places with hour-angles of 180 and 320. 



