and Voltmeter with a Long Scale. 



200 



Fig. 5 shows the form of the 220° polepieces without the 

 magnet. The coil is entirely enclosed in iron, and its three 



Fkr. 5. 



D-- 



-D' 



sides are active. In all other respects the arrangement is 

 precisely the same as the previous ones. 



Fig. 6. 



Fig. 7. 



■///// S////1 xnzzszzm 



/ 



/ 



igzzzzszza \nzs zz m 



\-* 



Let fig. 6 represent a vertical section through D D' in 

 fig. 5, and let fig. 7 be the movable coil. 



The lines of force cut the coil perpendicularly at all points. 

 Let I be the length of the periphery of the coil, r the radial 

 side, and n the number of turns in it. Considering one of 

 the horizontal sides of the coil with a current c flowing in it, 

 the moment of force acting at a distance x on an element 

 dx is 



= Hn ex dx. 



The moment acting on the vertical side is 



= Hnc y^ — Ar, 

 H being the intensity of the magnetic field. 



