On a new form of Amperemeter and Voltmeter. 205 



extensive area. The relative thickness of the gap shown 

 in the figure is much greater than it is in the actual 

 instrument. 



When this form was tried it was found to have some 

 defects on practical grounds ; the attachment and adjustment 

 of the moving system became a difficult matter. Once made, 

 however, an instrument having this form of magnetic circuit 

 is not at all a bad one, and of all forms it offers the longest 

 range. 



Fur. 1. 



The form of Magnetic Circuit first employed. 



The moving coil was rectangular in shape, and one side of 

 it — forming the axis around which it rotated — passed through 

 the central hole in B, while the opposite side moved in the 

 narrow air-gap. This form of magnetic circuit (fig. 1) is not 

 unlike that adopted by Mr. W. Hibbert in his apparatus for 

 a standard magnetic field, except that in mine the steel 

 magnet is eccentric *. 



The next form is shown in fig. 2 (p. 206). This is much more 

 easily manipulated, and the difficulty of adjusting the movable 

 system is considerably reduced. M is the steel magnet, A 

 and BD are the two wrought-iron poles. The frame sup- 

 porting the moving system is in this case mounted on the 

 polepiece A. 



* See Proc. Phys. Soc. vol. xi. p. 306. 



