334 Prof. Oersted on Electro-magnetism. [Nov. 



round the earth. It is true that this electrical effect will be weak 

 at each point, but the great extent of the electrical surface will 

 abundantly supply it. The length of the circle or electrical belt 

 is that of the periphery of the earth. The width of this belt extends 

 nearly as far as the vicissitudes of night and day during a revolution 

 of the earth. The width of this belt varies every day, since the 

 diameter of the circle around the poles of the earth, during night 

 or day, changes continually during several revolutions. For at 

 66° 32' from the equator, there is once a year 24 hours of day, 

 and 24 hours of night ; at 67° 18' there is a whole month of day 

 light, and a month in which the night continues without inter- 

 ruption, Sec. The mean width of this circle will extend but 

 little further than the polar circle ; for the violent changes which 

 impede the regular progress of the weather, and consequently 

 the regular effect of solar influence, exert great controul in the 

 countries near the poles. 



M. Ampere supposes that there is also an electro-magnetic 

 effect round the earth from the east to the west, but he is of 

 opinion that it belongs properly to the construction of the earth, 

 although its revolution is not without some effect; he supposes also 

 that there is no other magnetism of the earth, besides the imme- 

 diate effect of electro-magnetism. As to this last point, my 

 opinion also differs from that of the French philosopher. A body 

 capable of becoming magnetic cannot be surrounded with an 

 electrical current without receiving a magnetic charge. All 

 bodies are susceptible of magnetism to a certain extent, although 

 generally it is very slight when compared with iron. It follows 

 then necessarily from the electrical circulation round the earth, 

 that the earth itself becomes magnetic. 



Let fig. 14 represent a section of the terrestrial globe supported 

 by its two poles. Suppose + E in the electrical belt goes 

 from the east to the west, and of course — E from west to east, 

 — £ upon the surface of the earth goes then towards the north, 

 and + e towards the south ; but the contrary direction must take 

 place at the inferior side of this- belt. The globe S z N z here 

 represented as a nucleus surrounded by the crust which contains 

 the electro-magnetic belt, will become magnetic, and at n it will 

 possess the magnetic power which we find in that half of the 

 magnetic needle which turns towards the south. It is thus that' 

 the magnetic needle receives its direction by the magnetism of 

 the earth, and by the electro-magnetism of the surface. If the 

 magnetic nucleus derived its magnetism from the electro-mag- 

 netism of the surface, its north pole would repel the same end 

 of the needle that is attracted by the north side of the electro- 

 magnetic belt ; but as the lower plane produces opposite mag- 

 netism, the surface and the magnetic nucleus have the same 

 effect with regard to the needle. 



The intensity of action cannot be equal in the whole of the 

 eleGtro-magnetic belt of the earth, just as the effect of the sun is 



