730 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



The difference in phase as well as the change in the relative magnitude of the amplitudes of the 

 two phenomena with variation in length of period of vibration, is against the view that the diurnal 

 variation of terrestrial magnetism is entirely due to earth-currents. In fact most authorities Barlow 

 Airy, Wild, Lemstrom, Ellis consider it very doubtful whether the earth-currents can explain the diurnal 

 variation of terrestrial magnetism. 



We are not at present going to discuss fully the problem of the diurnal variation of terrestrial 

 magnetism, which will be reserved for a subsequent chapter; but I think we may say that in spite of 

 of the most elaborate researches into the laws of terrestrial galvanic currents, no one has been able to 

 show that these currents form the principal cause of the diurnal variation of terrestrial magnetism. 

 Moreover recent investigation on magnetic diurnal variations, especially by A. SCHUSTER( I ), von BEZOLD|'-| 

 and SCHMIDT has led to the result that the currents causing the diurnal variation must have their seat 

 above the surface of the earth. 



EARTH-CURRENTS AND MAGNETIC DISTURBANCES. 



143. Most investigators in the field of earth-currents since 1880, have confirmed the result of 

 Airy with regard to the connection between these currents and magnetic disturbances. It is in particular 

 Blavier who has got results essentially different from those of Airy. Lemstrom, Wild and Bachmetjew, 

 however, all agree with Airy, who considers the earth-currents to be the cause of magnetic disturb;] 

 Most investigators, however, consider that there are certain exceptions yet to be explained. 



Blavier, on the other hand, found that the earth-currents and magnetic disturbances are not related 

 in such a way that the earth-currents have produced the magnetic variations; but he takes rather the 

 opposite view that earth-currents are produced by the changes of magnetism. According to him the 

 magnetic disturbances were mainly due to extraterrestrial currents above the place, while the earth- 

 currents are produced by induction due to changes in the extraterrestrial currents. 



This assumption is based on the fact that from his records he found the amplitudes of the accidental 

 earth-current to be proportional to the rate of change which at the time considered is found for the 

 corresponding magnetic elements. 



Although Blavier, in a way, is certainly on the right track, I should consider it probable, in view 

 of the results of the other investigators, that he is giving his conclusions too great generality. It might 

 even be possible, as Blavier himself admits, that his induced currents are not altogether real earth 

 currents, but are partly currents induced in the cable system. Such currents, indeed, may have been 

 present and may have influenced the results so as to give the impression that the induction-relation 

 holds more general than it actually does. In order to find out whether the induction in the cable-system 

 exerted any real influence, Blavier made simultaneous observations over the same areas in underground 

 cables and in aerial lines. As the two curves thus obtained were identical, he thought himself justified 

 in concluding that the currents observed were due to actual earth-currents. 



Quite recently the question regarding the connection between earth-currents and magnetic distur- 

 bances has been treated by J. BOSLER ( 3 ), who has examined a number of disturbances recorded at Pare 

 Saint-Maur. He finds for the cases considered that the relation is such as would be expected if the 

 perturbing forces were directly due to the earth-currents flowing underneath the magnets. 



(') A. SCHUSTER: Phil. Trans, of the Roy. Soc. Vol. 180, p. 467, 1889. 



(*) W. VON BEZOLD: Sitzungsberichte der Kgl. Akad. d. Wissenschaften zu Berlin, 1897. 



( 3 ) J. BOSLER: Comptes Rendus, p. 342, 1911. 



