THE MAGNETIC MEASUREMENT OF CURRENT. 21 



on the magnet pole by the coil of wire must be equal to mh where 

 m is the strength of the pole and h is the intensity at the pole 

 of the field due to the coil. 



Consider, therefore, a magnet pole of strength m placed at the 

 center of the circular coil as shown in Fig. 16. This pole produces 

 a magnetic field of which the 



, m 



intensity at the wire is -^, ac- 

 cording to Art. 9, and the 

 lines of force of this field are 

 at right angles to the wire. 

 Therefore, according to Art. 

 13, the wire is pushed sidewise 

 (towards the reader in Fig. 

 1 6) with a force of 2xrZ 



m 

 X I X -T dynes where 2xrZ 



slim magnet 



Fig. 16. 



Circular coil of two turns with cur- 

 rent / flowing in it and with magnet 

 pole m at its center. 



is the length of the wire (Z 



being the number of turns of 



wire in the coil), and I is the strength of the current in the coil 



in abamperes. 



Now the force exerted on the coil by m is equal and opposite 

 to the force exerted on m by the coil. Therefore, disregarding 

 signs, we have 



mh = 27irZ X 



X 



or 



where h is the intensity at the center of the coil of the magnetic 

 field due to a current of / abamperes in the coil, r is the radius 

 of the coil in centimeters, and Z is the number of turns of wire 

 in the coil. 



