Royal Society. 209 



rendered magnetic by induction was first investigated, in 1829, by 

 Mr. Barlow j and also, by Mr. Charles Bonnycastle, Professor of 

 Mathematics in the University of Virginia -, it has since been amply 

 confirmed by the more elaborate analytical investigations of Pois- 

 son. But the result of all these inquiries, instead of affording 

 us clearer notions of the action of terrestrial magnetism, tended 

 rather to perplex and obscure our views respecting its nature and 

 operation. 



While our knowledge was in this imperfect and almost retrograde 

 state, a new light broke in upon us in the great discovery of Oersted, 

 which, by disclosing the intimate relation which electricity bears to 

 magnetism, must be regarded as forming a new era in the history of 

 this department of physical science. The operation of the tangential 

 force between a galvanic wire and a magnetic needle was pointed out 

 by the author, in a paper which was read to the Royal Society in the 

 year 1822 j and was still more fully examined by M. Ampere, who 

 extended the investigation to the law of the reciprocal action of gal- 

 vanic currents on one another ; and thence deduced a general theory 

 of magnetic action. 



Having established the general fact that the magnetism which is 

 induced on an iron ball resides only on its surface, and acts accord- 

 ing to the same laws as the magnetic influence of the earth, the 

 author was desirous of ascertaining whether he could succeed in imi- 

 tating the effects of terrestrial magnetism by distributing galvanic 

 currents round the surface of an artificial globe. This conjecture he 

 put to the test of experiment, by having a hollow wooden globe 

 constructed, sixteen inches in diameter, with grooves cut at all the 

 parallels of latitude distant by 10 from each other. Copper wires 

 were then laid in these grooves, and disposed so as to allow of the 

 transmission of a galvanic currrent in similar directions through the 

 whole system of these circular wires. This being effected, it was 

 found that a magnetic needle, properly neutralized, so as to be ex- 

 empt from all influence from the earth, and freely suspended in different 

 situations on the surface of this artificial globe, assumed positions ex- 

 actly analogous to those of the dipping-needle in different parts of 

 the earth. The author has no doubt that if the electrical currents 

 in this experiment could be increased indefinitely, the apparatus 

 might be made accurately to represent every circumstance of mag- 

 netic dip and direction actually observed in nature. 



It thus appears that all the phenomena of terrestrial magnetism 

 may be produced by electricity alone : for it is evident, that in place 

 of the needle employed in the experiment above described, the gal- 

 vanic needle of Ampere might have been substituted, to the complete 

 exclusion of the only magnetic part of the apparatus. 



The discovery of Seebeck, that heat applied to a circuit of metallic 

 conductors developes galvanism, and consequently gives rise to 

 magnetic induction, supplies another link in the chain of evidence, 

 that terrestrial magnetism is purely an electrical phenomenon, de- 

 riving its origin, during the diurnal revolution of the earth, from the 

 action of the sun's rays on successive portions of its surface, in d. 

 rections parallel to the equator. The probability, therefore, is now 



N. S. Vol. 9. No. 51. March 1831. 2 E much 



