Sec. 3-1] 



M. UISETIC TRANSDUCERS 



209 



moving coil magnetometer. A moving coil, Fig. (3-1)35, with n 

 turns and the cross-sectional area a ( = L W) is suspended so that it 

 can turn around an axis yy. A current / passes through the coil. If 

 the coil is placed in a magnetic field H in the direction of the coil 

 plane, a torque t will tend to rotate the coil against the restoring 

 y 



I 



Fig. (3-1)35. Moving- 

 coil magnetometer. 



G 



Fig. (3-1)36. Wire-loop magnetom- 

 eter. 



H 



H = 



force of the springs or of the suspension wires. For a small angular 

 displacement q? the torque is 



r = Ia?ifiH = Tccp 



where k is the spring constant. If the current is regulated so that 

 the same small angular displacement is always obtained (99 = const), 

 then the magnetic field can be found from 



hp C 

 an/ul I 



The apparatus constant C can be found from calibration in known 

 fields. 1 The method is applicable for magnetic field strengths be- 

 tween 10 and 10,000 oersteds. Traces of iron in the coil tend to 

 introduce errors. 



force upon a wire loop. A rectangular wire loop of the form 

 shown in Fig. (3-1)36 is suspended from the arm of a scale so that the 

 lower (horizontal) bar of the loop reaches into a magnetic field of the 

 strength H. If a current / passes through the loop, it will exercise a 

 force downward or upward which is 



F = blfiH 



1 H. S. Jones, Rev. Sci. Instr., 5. 211 (1934). 



