ELEMENTARY ELECTRICITY AND MAGNETISM. 5 



given test pole. A given magnetic field usually varies greatly 

 in intensity from point to point. Thus the magnetic field due to 

 a magnet A is much more intense at points near the poles of the 

 magnet than it is at points remote from the poles, for a test 

 pole is acted upon by a much greater force when it is near one 

 of the poles of A than when it is at points remote from both 

 poles of A. . 



The force, 3{, in dynes which acts upon a unit test pole when the 

 pole is placed at a given point in a magnetic field is adopted as the 

 numerical measure of the intensity of the magnetic field at the given 

 point. This force-per-unit-test-pole, ffl, is hereafter spoken of 

 simply as the intensity of the magnetic field at a point. 



The force F with which a magnetic field acts upon a test pole 

 of m units strength is m times as great as the force d{ with 

 which the field acts upon a unit test pole placed at the same 

 point. Therefore 



F = mW (2) 



in which F is the force in dynes which acts upon a test pole of 

 strength m when the pole is placed in a magnetic field of which 

 the intensity is 3f. 



The International Electrical Congress of 1900 adopted the 

 name gauss for the unit field intensity. 



Direction of a magnetic field at a point. The force with 

 which a magnetic field acts upon a north pointing test pole is 

 opposite in direction to the force with which the same field acts 

 upon a south pointing test pole placed at the same point. The 

 direction of the force with which a magnetic field acts upon a north 

 pointing test pole is adopted conventionally as the direction of the 

 field at the point. 



Homogeneous fields and non-homogeneous fields. A magnetic 

 field generally varies in intensity and in direction from point to 

 point. Such a field is said to be non-homogeneous. A magnetic 

 field which has everywhere the same intensity and the same di- 

 rection is called a homogeneous or uniform field. 



