68 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



Fig. 32. The force F with which the pole M repels the pole 

 m is given by equation (15), namely, FMmjr 2 , but the force 

 which acts upon the pole ;;/ is equal to mff where H is the 



M 



Fig. 32. 



intensity AT m of the magnetic field which is due to the agent 

 which is exerting the force on m, that is, where H is the inten- 

 sity of the field at m due to M. Therefore mH=F=Mm/r 2 , or 



-% . 07) 



in which H is the intensity in gausses of the magnetic field at a 

 point distant r centimeters from an isolated magnet pole of 

 which the strength is M. In the neighborhood of a north pole 

 the magnetic field is directed away from the pole and in the 

 neighborhood of a south pole the magnetic field is directed 

 towards the pole. This is evident when we consider that the 

 direction of the field is indicated by the direction in which a com- 

 pass needle would point, arrow-head being supposed to be on 

 the north-pointing pole of the needle. 



37. Representation of magnetic field intensity at a point by 

 means of a line. The magnetic field intensity at a point, like 

 the velocity of a fluid at a point, may be represented by a line 

 drawn in the direction of the field at the point, the length of the 

 line being such as to represent the intensity of the field at the 

 point to a convenient scale. 



Composition of magnetic fields. Consider two agents which 

 acting singly produce magnetic fields whose respective directions 

 and intensities at a point / are represented by the lines I and 2 



