582 MAGNETIC FIELD. 



the case, since the action of the magnet is equivalent to that of 

 the coil. 



These expressions will be very important in determining the 

 field of a magnet M by the action which it exerts on a second 

 magnet m of smaller dimensions, and it is useful to investigate 

 the importance of the terms of correction. 



In all cases the expressions for the couples A and B may be 

 put in the form 



A = cosa[i 

 B=- cos 



1154. Suppose, first, that the second magnet may be regarded 



as infinitely small in comparison with the distance to the centre 



L 



of the first. Putting p = , we have 

 K. 



P = 2p . P = "\p ) 

 n =. n2 n' =. n^ 



If the distance R, for instance, is four times the length of the 

 magnet, or R = 8L, it follows that /o 2 = 0-015625, /> 4 = 0-000244, and 

 the terms of correction are 



/= 0-031250, / 



q= -0-023437, q' = 0-000457. 



1155. When the length of the second magnet is not very small, 

 we shall assume that the two magnets are almost perpendicular to 

 each other, or almost parallel. 



If the magnets are almost perpendicular, the angles a and /3 

 may be neglected in the terms of correction. Putting ^-=A, we 



J_j 



have then 



