' PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. II. 443 



increases. While in the polar regions, such , movement of the storm-centre can be demonstrated, the 

 current-arrows in southern latitudes will turn a certain angle, clockwise or anti-clockwise, and always in 

 such a manner as to make it seem likely that it is produced by a movement of the whole perturbation- 

 area in the same direction as that in which the storm-centre in the polar regions moves. Thus, simultane- 

 ously with the movement of the storm-centre the whole pertubation-area in lower latitudes will move in 

 the same direction. As it appears from the character of the curves that the acting current-systems must 

 come near to the polar stations, while they must be comparatively distant from the stations in lower 

 latitudes, and also on account of the evident connection existing between the pertubations in high and 

 low latitudes, we have considered ourselves justified in drawing the following conclusion : 



During the negative polar storms, a current-system of some kind or other will be formed in the 

 pi'lar regions, the magnetic effects of which will be the primary cause of the perturbation-area formed. 



According to this, the magnetic pertubing forces in low latitudes must be considered for the most 

 part as very distant effects of this polar system of precipitation. The direct magnetic effect of this system 

 then, we believe would be the primary. There might moreover be imagined a number of secondary 

 effects, such as, in the first place, induced earth-currents, in the next, electric currents in the atmosphere 

 occasioned secondarily by the ionisation which, especially in the upper strata of the atmosphere, must be 

 thereby occasioned simultaneously. 



The question which next comes up is: How must this polar current-system be supposed to be 

 constituted? Here too, we believe the cause should be sought in corpuscular rays coming from the sun. 

 These rays, when the}' come under the influence of the magnetic field of the earth, will be drawn in in 

 zones round the magnetic axis. A single ray, considered by itself, will, if not under the influence of other 

 corpuscles, move in a spiral path in towards the earth, then turn, and leave the place in a similar 

 manner. We must thus imagine the corpuscular current as a whole, descending towards the earth in 

 paths that are more or less vertical, then turning when near the earth, and once more leaving it, unless 

 they are absorbed in the earth's atmosphere. How the rays, as a whole, will behave in the vicinity of 

 the earth, is a question that cannot be decided in advance. It is a problem that requires special treat- 

 ment. We have succeded in throwing much light upon the question by placing screens of various sizes 

 and shapes upon our terrella. These, when the terrella is irradiated with cathode rays, will cast shadows, 

 and from these shadows the course that the rays take near the earth can be directly measured. The 

 experiments will be described in Chapter IV of the present Part. The simplest assumption we can make 

 on the whole is that the rays in the vicinity of the earth turn round in an easterly or westerly direction. 

 The conditions round the auroral zone also show that during the negative polar storms, there are effects 

 like those of a horizontal electric current situated at a certain height above the earth. 



Upon this basis, we have tried to find out how near we are to the actual circumstances when we 

 assume that the polar current-system that is formed during a negative polar storm, can be replaced by 

 a current-system consisting of two vertical infinite branches, which are connected by a horizontal piece 

 of current. In Article 36 (pp. 102 & 103), we have made an estimate of how the horizontal forces vary 

 when we move from the storm-centre outwards along the transverse axis of the system. This showed 

 that as regards the horizontal forces, in the principal features even a quantitative agreement could be 

 reached between the observed forces and those calculated as the effect of this ideal system. In the pre- 

 ceding Article, we have also made a minute calculation of the magnetic effects of such current-systems. 

 A direct comparison of these areas with the observed pertubation-areas of the negative polar storms, 

 show the close agreement that exists here. In fig. 186, for the sake of distinctness, we have placed a 

 pertubation-area observed in one of the most characteristic elementary negative polar-storms in our 

 material, by the side of that of such a linear current-system. In the horizontal forces, the resemblance, 

 as we see, is striking. In the vertical intensity, the direction is also the same, but the observed forces 



