119 



ELECTRO-MAGNETISM. 



( THOSE capital experiments by which the science of magnetism has been re- 

 duced to the rank of a branch of electricity, by showing that all magnetical 

 < phenomena are merely effects of electrical currents modified by physical influ- 

 ences peculiar to certain substances, are of so very recent a date that they can 

 scarcely be considered as yet falling within the scope of scientific history. 

 Nevertheless, the important relations they bear to other parts of physics, the 

 high generality of the phenomena themselves, and especially their susceptibil- 

 ity of being reduced to mathematical analysis, require that they should not be 

 passed without some notice, even in a sketch so brief and rapid as the present. 

 Since, however, it is proposed in these volumes to enter very fully into the 

 details of the chief experiments which form the foundation of this part of elec- 

 trical science, it will be sufficient here to notice concisely the chief results, in 

 the order of their discovery, of those experimental investigations which may 

 be regarded as forming the basis of the division of the science now denomina- 

 ted electro-magnetism. 



At a very early period in the progress of electrical inquiries, indications 

 were observed of a relation existing between electricity and magnetism. 

 Ships' compasses had their directive powers impaired by lightning, and sewing- 

 needles were rendered magnetic by electric discharges passed through them. 

 . The influence of electricity over the magnetic properties of iron had been suf- 

 ' ficiently noticed to suggest to the clear and far-sighted mind of Beccaria a 

 notion, which can scarcely be called a vague one, of that theory of terrestrial 

 magnetism which may now be regarded as established on the basis of electro- 

 magnetical phenomena. 



No facts sufficiently clear and decisive to afford general conclusions were 

 produced until the year 1820, which was signalized by the greatest discovery 

 in physical science since the memorable invention of the pile. 



Professor Oersted, of Copenhagen, had promulgated certain theoretical views 

 the subject of the relations of electricity and magnetism in the year 1807, 



