PART III. EARTH CURRENTS AND EARTH MAGNETISM. CHAP I. 797 



Although we will not here enter upon a detailed treatment of this phenomenon, but will reserve 

 it for a subsequent chapter, in which the diurnal variation of magnetism at our four Norwegian stations 

 will be discussed, it will yet be natural, in connection with what has been said, even now to point to a 

 few circumstances regarding the diurnal variation of earth-currents, particularly as Dr. L. STEINER( I ), 

 upon the basis of Weinstein's curves, has drawn some very interesting conclusions. He finds that while 

 the diurnal variations in the magnetic X-component i. e. the force-component in the direction N S - 

 very closely follow the E W curve, so that the variations in the former may be supposed to be the 

 direct effect of the corresponding earth-current component, the deflections in the N S curve are approxi- 

 mately proportional to the rate of change of the component Y (in the direction E W). 



It will be seen that this is in the main the result at which we have here arrived by a conside- 

 ration of the earth-current conditions in Germany during the magnetic perturbations; and it would be 

 natural to look for an explanation of the diurnal variation similar to that of magnetic storms. In these 

 more slowly passing variations, however, other forces will exert an influence to a much greater extent 

 than in the briefer variations. The thermo-electric forces in the earth's surface may perhaps play a 

 very important part; and as STEINER suggests it may possibly be these currents that are mainly the 

 cause of X so closely following the E W curves. Of the other phenomenon, however, he gives no 

 satisfactory explanation, but remarks that "these connections as far as they are not due merely to 

 chance - - still await explanation". Our points of view naturally lead us to explain these conditions 

 in the following manner. 



Both ou,r observations and our experiments have shown us that broadly speaking a purely geo- 

 metric connection must always exist between the position of the sun and the situation of the systems 

 of perturbation. In other words, what has been said seems with undoubted certainty to show that the 

 earth will as a rule rotate in relation to an external corpuscular current-system with a more or less 

 fixed position in space. The strength of the current, especially during magnetic storms, may vary 

 within very wide limits; but its form has always proved to be approximately constant. I further assume, 

 as already stated, that from the entire surface of the sun, a comparatively regular radiation of corpuscle- 

 rays goes on, similar to the stronger and more irregular pencil-radiation of probably stiffer rays 

 from the regions of the sun-spots. 



This corpuscular field of radiation from the entire surface of the sun will now constantly surround 

 the earth, and it is obvious that its shape will in the main be the same as that which we have found 

 to be characteristic of the magnetic storms. When the earth now rotates in relation to this system of 

 rays with its approximately fixed position in space, earth-currents will be induced. 



The formulae necessary for the calculation of these are given in Article 155, equations 42 44. 



In this chapter we have also calculated the earth-current system that is induced by a polar system 

 of precipitation of the previously-described form (see Table CXXVI). 



Fig. 283 is a chart of the current-lines on the surface, answering to this; and we may here once 

 more draw attention to the peculiarity already pointed out, namely, that the direction of the current- 

 lines in medium latitudes such as those regions of Germany in which Weinstein made his observations, 

 is practically only N S. 



It further appears very distinctly from the experiments that the rays in the equatorial regions are 

 concentrated in such a manner that the main body of the ray-system swings round in front of the earth 

 and passes nearest just before noon (see fig. 219). As the greater number of the rays run here, this 

 system will in all probability also play an important part in this connection. . 



(>) Terr. Magn. XIII, p. 57. 



Birkcland. The Norwegian Aurora Polaris Expedition, 19021903. 101 



