PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. V. 615 



All the trials represented on the adjoining figure have been made with terrella No. 4 of a 

 diameter of 8 cm. or perhaps 7.8 cm. when without the phosphorescent coating, and weighing 977 gr. 

 The resistance of its magnetising coil is 1.72 . The magnetic momentum at 10 amperes magnetising 

 current was 27 200. 



The first experiment (see fig. 223) was made with a discharge- current of 20 milliamperes under 

 pressure of about 0.005 mm ' a "d with a magnetising current of 21 amperes. 



In addition to the equatorial ring, discharges will be seen from the northern polar zone. This 

 polar discharge is easily produced if there are any sharp points or unevennesses, but, on the other hand, 

 it is difficult to obtain it when the surface is smoothly polished, as was the case in all the other 

 experiments represented on the plate. 



The picture No. 4 has been placed beside this for the sake of comparison. It is a view of the 

 sun during an eclipse, May 1701, 1901. The picture is drawn by H. R. MORGAN from the negatives. 

 I will, later on, by the aid of points in the magnetic polar regions, both N. and S., produce a more 

 perfect example corresponding to photograph No. i, as this is obviously of great interest. 



Photographs 2 and 3 are from an interesting experiment seen from the side and from above, in 

 which the pressure was brought as in the first experiment, but the discharge-current was only 2 or 3 

 milliamperes and the magnetising current 26 amperes. 



The tension was 1500 volts before the magnetisation of the spherical cathode, and the radiation 

 from it could be seen to take place evenly from the entire surface of the sphere. After the magnetising 

 current was put on, the tension sank immediately to 600 volts and the radiation then took place only 

 from the equatorial regions of the spherical cathode. This could be plainly observed from the minute 

 glowing spots from which the rays issued, near the metal ball's equator. 



In the experiment represented in photograph No. 5 the pressure was as before, the magnetising 

 current 21 amperes, and the discharge-current 3 milliamperes. 



Photographs 6 and 7 are from an experiment with a magnetising current of only 2 amperes. The 

 ring is seen from the side and from above. Pressure 0.02 mm. and the discharge-current was 5 milli- 

 amperes. It is the low magnetising current that occasions the ring to be broad and small in extent. 

 A dark band is plainly visible between the magnetic sphere and the ring. It has happened on several 

 occasions that the luminous ring has been divided into two concentric rings by a dark circular band. 



We can find the conditions for electric radiation's getting out towards infinity from the surface of 

 a magnetic sphere. 



Suppose a magnetised sphere is giving out electric radiation of some kind. In the regions near 

 the poles the radiation will be able to get out by passing nearly along the lines of magnetic force. 



For rays in the magnetic equator, however, the magnetic force is perpendicular to the orbit of 

 the ray-particle, and unless certain conditions are fulfilled the radiation will not be able to emerge in 

 this place. 



It will be of interest for a number of questions in cosmic physics, to find the exact conditions 

 for rays in the place of the equator emerging into space. 



Let R and cp be polar co-ordinates in the plane of the equator with the centre of the sphere 

 as origin. 



We suppose the magnetic force to be perpendicular to the plane of the co-ordinates, and outside 

 the sphere given by the relation 



