728 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



An extensive series of registerings were undertaken in Germany on two long lines, one from 

 Berlin to Dresden, 120 km. and the other from Berlin to Thorn, 262 km., and are treated by B. WEIX- 

 STEIN ('). Continual photographic records were kept from 1893 97 at Pare Saint Maur, and in recent 

 years earth-current registerings have been made in Java by W. VAN BEMMELEN^). 



STRENGTH AND DISTRIBUTION OF EARTH-CURRENTS. 





141. In spite of the great amount of work done on the subject, the earth-current problem is still in a 

 somewhat unsatisfactory state. We are still very far from having attained a full comprehension of the 

 various causes producing the galvanometer deflections. 



The deflections observed are only to be considered as the algebraic sum of deflections due to a 

 great variety of causes, some of which are due to experimental arrangements, and even the true earth- 

 currents may be summed up from a very different origin. 



Wild, Blavier and Weinstein found that on an average the electromotive force between two 

 points in a certain direction is proportional to their distance, and Wild estimates that on undisturbed 

 days the electromotive force per kilometre is of the order Viooo v lt- Batelli finds 0.00068 volts per 

 kilometre along the magnetic meridian, and 0.00081 normal to it. 



During perturbations we shall find much greater values, Wild has found values up to 0.05 volts 

 per km., and at certain moments during the disturbance of September, 1859, the electromotive force in 

 telegraph-lines in France obtained values of about i volt pr. km. In 1881, PREECE found, in English 

 telegraph-lines, 0.3 volts per km. 



Some attempts at comparing simultaneous observations at various places were made by Lemstrom, 

 who coordinated his own observations for Sodankyla with those of Wild from Pawlowsk. 



He found that in the greater number of cases the conditions at the two stations were similar, 

 that great disturbances at the one station were accompanied by great disturbances at the other; but 

 there were also cases where no similarity was found, and LemstrOm concludes that besides the more 

 universal currents there are a number of quite local ones which are strong at the place but soon die off. 



He also makes an interesting comparison of the absolute magnitude of earth-currents at the two 

 places and finds as the average of 24 term-days that the amplitude at Pawlowsk is 0.0008 v l'/krn. and 

 for Sodankyla 0.06 vol t/km., or corresponding amplitudes are 75 times as large at Sodankyla. From 

 this rapid increase in the earth-currents towards the arctic regions Lemstrom was led to the suggestion 

 that probably there is a maximum zone of earth-currents similar to the auroral zone. 



One point on which most authorities seem to agree is that the earth-currents at a certain place 

 mostly run along a certain line of direction, either in the one direction or in the opposite. To this 

 circumstance it is to some extent, at any rate, due that the earth-currents will run along the lines in 

 which the earth's conductivity is greatest. In addition to this there are other reasons, e. g. an eventual 

 marked direction of the electromotive force, which causes certain marked directions to be found in 

 various districts. 



C. V. Walker has thought he could show that this constancy of direction was not due to local 

 causes, as he found the direction to be about the same for various places in England, viz: NE SW. 



Wild in Russia, Blavier in France, Batelli and Palmieri in Italy, and Bachmetjew in Bulgaria, also 

 found more or less the same direction NE SW; but Weinstein in Germany found it NNW SSE 

 and Lament almost E W in the neighbourhood of Munich. 





(!) B. WEINSTEIN: Die ErdstrOme im Deutschen Reichstelegraphengebiet und ihr Zusammenhang mil erdmagnetischen Er 



scheinungen. Braunschweig, 1900. 

 ( 2 ) W. VAN BEMMELEN: Koninklijke Akademie van Wetenschappen te Amsterdam, 1908. 



