382 MAGNETIC INDUCTION. 



say, that each of the volume-elements tends to move towards points 

 where the force is a minimum, and the movement of the whole of 

 the system is determined by this tendency of each element. 



Consider, for instance, the field produced by the opposite poles 

 of two identical magnets, or by the two poles of a horse-shoe 

 magnet, or more simply the field of two equal masses of opposite 

 signs (Fig. 34). 



In the centre of the figure O, at an equal distance from the two 

 magnets, the value of the force is a minimum in reference to the 

 diametrical line AA', and a maximum in reference to a direction Oy 

 perpendicular to the former. A small isotropic magnetic sphere, 

 which can only move along the right line O^, moves towards the 

 point O when it is in stable equilibrium; a diamagnetic sphere in 

 the same conditions would be in unstable equilibrium at the point O, 

 and would tend to move away to an indefinite distance. Even if this 

 sphere were absolutely free, and situate on the right line Ojy, at a 

 small distance from O, it would move away from this point along the 

 line Oy (that is, perpendicularly to the lines of force) , for that is the 

 direction in which the force varies most rapidly. 



402. A long magnetic needle, movable about the point O, would 

 set parallel to the line of the poles AA' in stable equilibrium ; each of 

 the volume-elements would tend towards points where the force is a 

 maximum. 



A diamagnetic needle, on the contrary, would be in stable equi- 

 librium in a direction perpendicular to the line of the poles. 



The needles set then parallel, or transversely to the line joining 

 two opposite poles, according as the coefficient of magnetisation is 

 positive or negative. Hence the names paramagnetic, or diamagnetic, 

 given by Faraday to bodies belonging to the first or second class. 



403. We have seen that even in a uniform field, a magnetic 

 needle places itself parallel to the lines of force, and on the other 

 hand the different elements tend towards points where the force is 

 a maximum. 



When the two kinds of actions are concordant, as in the pre- 

 ceding case, the position of equilibrium can be easily determined ; but 

 it may happen that the tendency of each element to move towards 

 the maxima of force may have the result of bringing the system into 

 a direction which is not parallel to the lines of force. The position 

 of equilibrium depends, in that case, on the conditions of experi- 

 ment. Suppose, for instance, a series of identical soft iron needles 

 arranged perpendicularly, and at equal distances from each other, 

 on a non- magnetic rod, and let this system be placed between the 



