420 CURRENTS AND MAGNETIC SHELLS. 



PART IV. ELECTROMAGNETISM. 



CHAPTER I. 

 CURRENTS AND MAGNETIC SHELLS. 



442. OERSTED'S EXPERIMENT. Older experiments on electrical 

 discharges had already shewn that the passage of a current in a 

 conducting wire could modify the magnetism of a steel needle. 

 These phenomena, to which only small importance was attached, 

 were a first indication of the relations which existed between elec- 

 tricity and magnetism. It is only since 1820, in consequence of 

 (Ersted's experiment, that the existence of these relations has been 

 made completely evident by the immortal researches of Ampere. 



When a straight conductor traversed by a current is brought 

 near a magnetised needle, the needle is, in general, deflected 

 from its position. In order to explain in all cases the somewhat 

 complicated effects which are observed according to the relative 

 positions of the magnet and the current, Ampere gave a very simple 

 rule : Suppose an observer placed in the wire in such a manner 

 that the current enters at his feet and emerges at. his head ; the 

 observer, turning his face to the needle, always sees the North pole 

 turn towards his left, which for the future we shall call the left of 

 the current If the needle were freed from the action of the Earth, 

 and of any other action than that of the current, it would set at right 

 angles with the current. 



443. MAGNETIC FIELD OF A CURRENT. The fundamental fact 

 which results from (Ersted's experiment, is that an electrical current 

 of any given form creates about itself a true magnetic field. 



This field possesses all the properties observed in an ordinary 

 magnetic field, for the actions which it exerts at any point on 

 equal magnetic masses of opposite signs are equal and directly 



