BODIES WHICH ARE FEEBLY MAGNETIC OR DIAMAGNETIC. 659 



the same horizontal plane, and forming an almost astatic system, 

 comprise between them the two coils. 



The astatic system, which has a small directive force, is placed 

 in the plane of symmetry perpendicular to the axis of the coils, and 

 should not be acted upon by them ; this absolute neutrality is 

 difficult to obtain, but the residual effect is counterbalanced by a 

 small auxiliary coil. The needles are contained in a small copper 

 box, which acts as damper. 



The current is passed in contrary directions in the two coils, and 

 by the mirror method the position of equilibrium is observed when 

 the two bars occupy the position (I), and then the position (II) ; 

 it is observed that the deflection with bismuth is opposite that which 

 would be obtained with a magnetic body. 



We may, with Weber, increase the deflections by the method of 

 multiplication, by giving the two bars a backward and forward motion 

 in harmony with the oscillations of the needle. 



The coils being so long that the field may be considered uniform 

 within the limits of the swing, there are no other currents than those 

 due to the displacement of the bars in respect of the needles. Ex- 

 periment show's that this effect is almost null even with copper bars. 



1220. EXPERIMENTAL METHODS. In order to determine the 

 magnetic susceptibility , of feebly magnetic or diamagnetic bodies, 

 recourse is had to the action of a non-uniform field. The variation 



of energy of a small volume u, at a point where the intensity of the 



uk 

 field is (f>, is equal to d<j> 2 , and the effort with which it tends to 



2 7 -x / n 



move along a path ds is expressed by - (394). 



ty 2 * 



The differential - - only depends on the form of the field : but 

 OS 



it is unnecessary to get it by experiment, for it disappears in the 

 ratios. We might, moreover, determine it by an examination of 

 various points of the field by Verdet's method. 



<)<i 2 



If the field is produced by a current, the value of - - is the 



OS 



product of the square of the intensity I of a current, by a factor 

 depending on the shape of the coil. In a series of successive 

 experiments, the current should be constant, or else its variations 

 be allowed for. 



U U2 



