THE MAGNETISM OF IRON. 67 



point in a definite direction, and the tendency of the needle to 

 point in a definite direction is due to the fact that equal * and op- 

 posite forces FF, Fig. 31, are exerted on the two poles of a 

 magnet by the magnetic field, that is to say, a magnetic field is a 

 region in which a magnet pole is pulled in a definite direction 

 (the direction of the field in the case of a north pole, the oppo- 

 site direction in the case of a south pole). The force //, in 

 dynes, which acts upon a unit magnet pole when it is placed at 

 a given point in a magnetic field is adopted as the numerical 

 measure of the intensity of the field at the point. This force-per- 

 unit-pole H is hereafter spoken of simply as the intensity of 

 the field. The unit of magnetic field intensity (one-dyne-per- 

 unit pole) is called the gauss. 



Complete expression for the force with which a magnetic field 

 acts on a magnet pole. The force with which a magnetic field 

 acts upon a magnet pole of m units strength is m times as great 

 as the force H with which the field acts upon a unit pole placed 

 at the same point. Therefore 



F=mH (16) 



in which F is the force in dynes which acts upon a magnet pole 

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

 intensity is H gausses. 



Uniform and non-uniform fields. A magnetic field is said to 

 be uniform or homogeneous when it has at every point the same 

 direction and intensity, otherwise, it is said to be non-uniform or 

 non-homogeneous. The earth's magnetic field is in many places 

 sensibly uniform throughout a room. The magnetic field sur- 

 rounding a magnet is non-uniform. The magnetic field surround- 

 ing an electric wire is non-uniform. 



36. Direction and intensity of the magnetic field surrounding an 

 isolated magnet pole. Consider the poles of two magnets of 

 which the strengths are M and m, respectively, as shown in 



* This statement refers to the case in which the field is uniform, as will be seen 

 later. 



