Terrestrial Magnetism. — "On the question lohether the internal 

 maynetic field, to which the diurnal variation in terrestrial 

 magnetism is partly ascribed, depends on induced currents'' 

 By Miss Annie Van Vleuten. (Communicated by Dr. J. P. 

 VAN DER Stok). 



(Communicated in* the meeting of June 30, 1917.) 



1. In 1889 Schuster ^) suggested that the diurnal magnetic variation 

 might be explained by a system of electric currents exterior to the 

 earth, and currents induced within the earth by the former system. 



Assuming that the conducting power of the earth increases with 

 the depth, he found the results of his analysis of the horizontal 

 and vertical forces in harmony with that hypothesis. 



2. In 1902 Fritsche -) starting from other data expanded the 

 potential of the diurnal field of variation in a series of spherical 

 harmonics, and in 1912 Steiner ^) examined whether these numbers 

 contained a corroboration of Schuster's idea. 



It appeared that the mutual relations of the two parts of the 

 potential corresponding to the external and the internal field did 

 not cori-espond to the theory. 



By taking into account also the magnetic permeability he could 

 obtain a reasonable agreement for some of the terms but in general 

 the above mentioned supposition was 7iot confirmed. 



3. In 1913 Fritsche^) published new values for the potential 

 coefficients of the diurnal variation, using the same material as in 

 J 902, except that of the polar stations, which seemed less reliable. 



If we repeat Steiner's investigation by means of these values, 

 these numbers are no more than the coefficients of 1902 in accordance 

 with the supposition, that the internal field is caused by induced currents. 



1) The Diurnal Variation of Terr. Magn. Phil. Trans. Vol. 180 (1889) A. 



2) Die Tagliche Periode der Erdmagnetischen Elemente. St. Petersburg 1902. 



*) Die Bestimmung der Elemente des Erdmagnetismus und ilirer zeitlichen Aende* 

 rungen. Riga 1913. 



*) Ueber die tagliche Variation der erdmagnetischen Kraft. Met. Z.schr. 29 (1912). 



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Proceedings Royal Acad. Amsterdam. Vol. XX 



