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XXVII. — On Earth-Currents during Magnetic Calms, and their connection with 

 Magnetic Changes. By Bai-four Stewart, M.A., F.R.S. 



(Read 6tli April 1863.) 



I have already endeavoured*! to prove that Aurorse and Earth-currents, far 

 from producing magnetic disturbances by their direct action, are themselves only 

 secondary or induced currents, generated by those small but abrupt changes of 

 the earth's magnetism which constitute such disturbances. The proof of this 

 statement was in the first place derived in a general manner from the fact that 

 during the notable magnetic storm of August and September 1859, all the ele- 

 ments of the earth's magnetism at Kew remained for many hours on one side of 

 their normal positions, while, on the other hand, the earth-currents observed 

 during that time by Mr C. V. Walker had their direction reversed every two or 

 three minutes.* 



In the next place it was shown that the earth-currents, which occurred simul- 

 taneously Avith a singularly abrupt and isolated disturbance, recorded by photo- 

 graphj^ at Kew, were of a character which would favour the induction hypo- 

 thesis rather than that of direct action.f 



Professor W. Thomson has likewise found by calculation, that the electro- 

 motive force, induced by variations of terrestrial magnetism, is probably com- 

 parable in amount with that which manifests itself in earth-currents. f 



Since the publication of these views, Mr Walker has extended his inquiries, 

 and has communicated to the Royal Society of London an account of some earth- 

 currents which he had observed by the aid of a sensitive galvanometer during a 

 period of magnetic calm.]: It is these observations which I propose to discuss in 

 the present communication. 



Mr Walker has been extremely careful in determining that all his ob- 

 served currents were in no sense atmospheric, but were true and proper earth- 

 currents, whatever may have been their cause. In order to accomplish this, he 

 had a telegraphic wire of 67 miles in length, connected with the earth at one of 

 its extremities, the other being insulated. On this wire he made many observa- 

 , tions during various hours of different days, but always failed in obtaining a 

 current unless when both ends Avere connected with the earth. 



* Phil. Trans, for 1861, p. 423. f Ibid, for 1862, p. 621. + Ibid, for 1862, p. 203. 



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