320 



Prof. S. P. Thompson. On the [Feb. 14, 



or steel whose magnetism is to be tested is laid, in the "first 

 position " of Gauss f " end on "), within its magnetising coil, a second 

 coil being added on the other side of the suspended needle to 

 compensate the action due to the. coil alone. At a certain distance 

 along the platform, and having its centre upon the line of the plat- 

 form groove, is set a magnet — called by Hughes the "compensator" 

 — of considerable magnetic moment. The compensator is so pivotted 

 as to be capable of being rotated round a vertical axis through its 

 centre over a scale ; and, according to the original design of the 

 instrument, the compensator and scale are provided with an arrange- 

 ment whereby they may be shifted along the platform, so that they 

 can be made to approach nearer to the suspended needle when a more 

 powerful compensation is desired, or removed further away when a 

 more delicate magnetic force is to be compensated. In practice the 

 balance was obtained by fixing the central pivot of scale and com- 

 pensator at a distance of 30 or more centims., and turning the 

 compensator upon its pivot until its magnetic force on the suspended 

 needle, or rather its resolved part in the axial line of the platform, 

 was exactly equal and opposite to that of the piece of iron or steel. 



Fig. 1. 



[2.] Theory of the Magnetic Balance. — In fig. 1, let the line AX 

 represent the central line of the platform of the balance lying 

 magnetically east and west. The point M is the centre of the 



