Sec. 3-1] 



MAGNETIC TRA SSDCCERS 



183 



moving the coil in or out of the field, it can be rotated through an 

 angle of 90° (AO = O) or 1S0° (A0> = 20)). 



Both methods are applicable for fields as small as 10 -2 oersted. 

 For smaller fields the coil dimensions become unwieldy or the output 

 voltage becomes too small for convenient measurement. 



Axis of 

 ro tot ion 



Coil axis 



Direction 

 of H 



Fig. (3-1)3. Rotating search coil with slip rings for a-c 

 output. 



The continuous conversion of magnetic fields into electric signals 

 is accomplished with an arrangement such as that shown sche- 

 matically in Fig. (3-1)3. If the coil Lis placed in a uniform magnetic 

 field H and is rotated continuously with the angular frequency a>, a 

 sinusoidal voltage will appear at the output with an rms value 



najuHco 



E = 



Direction of H "^S-r- x 



>^ 



A/2 



The ends of the coil are connected to slip rings, and contact is made 

 with brushes. The axis of rotation must be perpendicular to the field 

 direction. The rms value or peak 

 value of the output is independent 

 of the direction of the magnetic 

 field normal to the axis of rotation. 

 However, the phase angle between 

 the instantaneous value of the a-c 

 voltage output and the position of 

 the rotating coil furnishes infor- 

 mation about the field direction; 

 the instantaneous voltage is a 

 maximum when the coil plane is 

 parallel to the field lines. If a d-c output is desired, a rectifying 

 commutator can be used as shown in Fig. (3-1)4. This arrangement 



Commutator 



Fig. (3-1)4. Rotating search coil with 

 commutator for d-c output. 



