PART III. 



THE ELECTROMAGNETIC FIELD. 

 CHAPTER XL 



ELECTROMAGNETISM. 



210. Magnetic Force due to Linear Current. The dis- 

 covery was made by Oersted*, in 1820, that if a linear circuit be 

 traversed by an electric current, the space in its neighborhood 

 constitutes a field of magnetic force. The nature of the forces of 

 the field was completely investigated by Amperef, who found that 

 they were of the same nature as if they proceeded from permanent 

 magnets. They accordingly have a potential, which, with its first 

 derivatives, is continuous and vanishes at infinity, and which 

 satisfies Laplace's equation at all points outside of the conducting 

 wire, supposing that a single homogeneous medium is present. 

 We have however seen that a single-valued, or uniform function 

 having all these properties vanishes everywhere. Accordingly the 

 magnetic potential due to a current is not uniform. 



* Oersted, Experimenta circa effectum Conflictus Electrici in Acum Magneticam, 

 Copenhagen, 1820. 



t Ampere, " M&noire sur la theorie mathematique des ph^nomenes Slectro- 

 dynamiques, uniquement deduite de 1'experience. Gilbert's Ann. 67, 1821 ; Mem. 

 de VAcad. t. 6, Ann. 1823. 



