288 MAGNETISM 



The galvanometer throw gave Hy. To find 77- use was made of 



the fact that = -7 ^> since each is equal to -, j-, where V is 



ax ay il.nl if 



the magnetic potential. The coil was placed at o with its plane 

 perpendicular to H,, and was moved a given small distance dy 



along oy, and the throw gave r dy 



By these experiments, then, H y r 1 could be found. Pre- 



liminary experiments were made to determine the values of 



dU dH 



H y and ^ or -T * at various points along ox, and the point 



where the product was a maximum was fixed as at o. Since, there- 

 abouts, it varied slowly, exact adjustment of the body at o 

 not necessary. Further, the force along ox was practically con- 

 stant for a small displacement along ox. In the experiment! the 



219. 



t torsion fibre, r torsion rod, 6 body, EE electro-magnet, 

 * scale, p pointer, m microscope 



displacement due to magnetic action was never more than (H5 cm. 

 It was not necessary, then, to restore the body to iN original 

 position, when the deHecting force acted. It was sufficient to read 

 the deflection. This was done by viewing through a microscope a 

 pointer on the other end of the torsion arm which moved over a 

 scale. The arrangement is represented diagram matically in Fig. 

 219, where certain details for measuring the moment of inertia are 



