MAGNETIC EFFECT OF ELECTRIC CURRENT. 



ID/ 



number of turns of wire in the coil. The field H is parallel to 

 the top and bottom edges, or limbs, of the coil, and at right 

 angles to the two side limbs of the coil. The right-handed limb 

 of the coil in -Fig. 7 1 is pushed forwards (towards the reader) and 

 the left-handed limb of the coil in Fig. 71 is pushed backwards 



two fine suspending 

 wires 



fop rieiP 



Fig. 71. 



(away from the reader), and the force in each case is equal to 

 Za x fix f, according to equation (28), / being the strength 

 of the current in the coil in abamperes. It is evident that the 

 total force action on the coil is a torque tending to turn the coil 

 about the axis of suspension ; the value of the torque may be 

 obtained by multiplying the force acting on each limb of the coil 

 by its lever arm /2 and adding the two results together, which 

 gives 



T= abZIH (38) 



in which T is the torque in dyne-centimeters tending to turn the 

 coil and / is the current in the coil in abamperes. 



If the rectangular coil is allowed to turn through an angle 6 

 about the axis of suspension in Fig. 71, then only the component, 

 H cos 6, of the field will be effective in producing torque as 

 shown in Fig. 72, and equation (38) will become 



T= abZIHcos ' (39) 



