82 



EXPLORATION GEOPHYSICS 



which it is placed. (See Figure 17.) NS represents a magnet placed at an 

 angle a to the direction of the field, and of pole strength m. 



Assuming a magnetic field of strength H, the N pole of the magnet 

 experiences a force, -{-Hm, and the south pole a force, — Hm, the two 

 forces giving rise to a couple whose turning moment is equal to either 

 force multiplied by the perpendicular distance (AN) between them. 

 AN = NS sin a;NS = 2l;andM = 2 ml. 



Therefore, the magnetic couple = 



Hm (AN) = Hm (NS) sin a = MH sin a. 



(27) 



This couple becomes zero when the magnet assumes a position parallel to H. 



Fig. 17. — A magnet free to turn placed in a 

 uniform magnetic field. 



This expression for the couple acting on the 

 parts : H, the disturbing field ; a, which defines the 

 and m (NS), a characteristic of the magnet itself, 

 familiar magnetic moment, M, of the magnet. 



In the case of an ideal magnet consisting of 

 previous consideration, the magnetic moment was 

 times distance between poles : or 2 ml. However, 

 moment from the expression 



couple = MH sin a 



needle consists of three 

 position of the magnet ; 

 This last quantity is the 



two poles, used in our 

 defined as pole strength 

 a definition of magnetic 



(28) 



does not depend on the idea of an ideal magnet. Any magnet may be sus- 

 pended in a magnetic field, the couple required to maintain it in a given 

 position measured, and its M, therefore, determined. 



If the magnet is maintained at right angles to the magnetic field, a == 

 90° and the couple = MH. From this, another definition of the moment 

 of a magnet can be written : the magnetic moment, M, is the couple required 

 to maintain a magnet at right angles to a magnetic field of unit strength. 



