CHAPTER XIII. 



THE MAGNETIC FIELD PRODUCED BY ELECTRIC CURRENTS. 



Experimental Basis. 



480. So far the subjects of electricity and magnetism have been developed 

 as entirely separate groups of physical phenomena. Although the mathe- 

 matical treatment in the two cases has been on parallel lines, we have not 

 had occasion to deal with any physical links connecting the two series of 

 phenomena. 



The first definite link of the kind was discovered by Oersted in 1820. 

 Oersted's discovery was the fact that a current of electricity produced a 

 magnetic field in its neighbourhood. 



The nature of this field can be investigated in a simple manner. We 

 first double back on itself a wire in which 

 a current is flowing. It is found that no 

 magnetic field is produced. 



Next we open the end into a small 

 plane loop PQRS. It is found that at dis- 

 tances from the loop which are great com- 

 pared with its linear dimensions, such a 

 loop exercises the same magnetic forces as 

 a magnetic particle of which the axis is 

 perpendicular to the plane PQRS, and the 



moment is jointly proportional to the strength of the current and to the area 

 PQRS. The single current flowing in the circuit OPQRST is obviously 

 equivalent to two currents of equal strength, the one flowing in the circuit 

 OPST obtained by joining the points P and 8, and the other flowing in the 

 closed circuit PQRSP. The former current is shewn, by the preliminary 

 experiment, to have no magnetic effects, so that the whole magnetic field 

 may be ascribed to the small closed circuit PQRS. 



(2) 



FIG. 118. 



