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



Fig. 216 shows some results with the small terrella after the removal of the large screen through 

 the equator. 



Nos. i, 2 & 3 were all taken from a position with an hour-angle of 90. In No. I the magnetic current 

 was 10 amperes, in No. 2 there was no magnetisation, and in No. 3 the current was 20 amperes. The 

 full development of the polar ring of precipitation in No. 3 will be observed. 



In Nos. 4 9, the arrangement was the same, except that the magnetising current was 10, 20 and 

 30 amperes respectively for Nos. 4 and 7, Nos. 5 and 8, and Nos. 6 and 9, and the tension 2700, 2600 

 and 2700 volts respectively. 



Nos. 4 6 were taken from a position with an hour-angle of 270, and Nos. 79 from a position with 

 hour-angle 235 and declination 24. The positive precipitation on branch 2 of the star-shaped screen is 

 seen, whereas no positive precipitation appears on branch 3. Some experiments where made without 

 photographing, for the purpose of studying this circumstance more carefully; and it then appeared that 

 at the end of the positive side of branches 4 and 5, precipitation also occurred on our tiny terrella. 

 When this result is also compared with that obtained when there was a large equatorial screen, it will 

 be understood that it can hardly be only the rays that come in right across the polar regions of the 

 terrella that produce positive precipitation. 



ON THE SIZE OF THE POLAR RING OF PRECIPITATION. 



115. We will now pass on to describe experiments that were made for the purpose of determining how 

 the size of the rings of polar precipitation was dependent upon the magnetising of the terrella and the 

 magnetic stiffness of the cathode rays employed. The intention of the experiments was to procure a basis 

 for the judgment of the magnetic flexibility of the corpuscular rays coming from the sun and producing 

 aurora and magnetic disturbances upon the earth in the manner we have supposed them to do. 



In the experiments from which the photographs in fig. 217 were taken, the discharge-current in 

 every case was about 25 milliamperes, and the pressure in the discharge-tube 0.046 mm. The tension 

 difference between anode and cathode was 1800 volts in the experiments represented in the first and second 

 rows, and it went from 1800 to 1700 volts in those in the third row. The tension remains comparatively 

 constant here, because the pressure was so high that the amount of gas disengaged during the experi- 

 ment did not alter the conditions as much as it does when the pressure is small to begin with. 



The magnetising current in the three experiments was respectively 10, 20 and 30 amperes. 



The position of the terrella -- No. 4 -- was unchanged during the three experiments, this being 

 with the magnetic axis horizontal and at right angles to the central line to the cathode. The magnetic- 

 south pole had an easterly hour-angle of 270, and photographs i, 4 and 7 were taken from a place 

 outside with the same hour-angle, photographs 2, 5 and 8 from a place with an hour-angle of 180, and 

 photographs 3, 6 and 9 from a place with an hour-angle of 90. 



In fig. 218 there are 9 similar photographs from 3 experiments in which the discharge-current 

 was again about 24 or 25 milliamperes throughout, and the pressure in the discharge-tube about 0.008 

 mm. The tension in the three experiments was respectively 2400 volts, from 2400 to 2300, and from 

 2500 to 2300 volts, while the magnetising current was 10, 20 and 30 amperes. As will easily be under- 

 stood, our endeavours were aimed at keeping the tension constant in each series of experiments; in the 

 first series the tension aimed at was about 1800 volts, and in the second series about 2400 volts. 



From the two series of photographs answering one to 1800 volts and the other to 2400 volts, we 

 find in the first place that the stiffer the rays employed and the less the magnetisation of the terrella, 

 the larger are the polar precipitation-rings. The idea originally was to magnetise the terrella so strongly 

 that the polar precipitation-ring would acquire a spherical diameter of 45, very much as one imagines the 



