THE MAGNETIC MEASUREMENT OF CURRENT. 23 



direction of the resultant field R. Now tan 4> = from Fig. 

 1 8. Therefore, using - - for h and solving for 7, we have: 



rH' 



I in abamperes = - ^ -tan 



or 



5rH' 

 I in amperes = ^ tan 



(i) 



(2) 



1 6. Intensity of the magnetic field at any point in the axis of 

 a circular coil. Consider the point m, Fig. 19, in the axis of a 



Fig. 19. 



circular coil CC of radius r centimeters; the distance of m 

 from the plane of the coil being d centimeters. Imagine a 

 magnet pole of strength m to be placed at m. This pole pro- 

 duces at C a magnetic field of which the intensity is -= 



( = J. The component of this field which is parallel to 



the axis of the coil pushes radially outwards on each part of the 

 coil, tending only to spread the coil. But the component of 



which lies in the plane of the coil, namely, 



