Sec. 3-1] 



MA GNE TIC TEA N$D UCERS 



185 



strength can be observed, but such times must be large compared 

 with the period of rotation. The sensitivity varies widely with the 

 construction and is in general between 0.1 and 10 mV/oersted. An 

 accuracy of better than 0.1 per cent can be obtained. 



The general requirements for the conversion of magnetic fields 

 into electric voltages by means of oscillating coils are expressed in 

 Eqs. (2) and (5a, b, and c). In some oscillation transducers, the angle 

 between the field lines and the coil axis changes with time or is never 



I Coil axis 



Coil frame F 



Fig. (3-1)6. Oscillating search coil, schematic diagram 

 [from J . Grosskowski, J. iSci. Instr., 14, 335 (1937)]. 



zero degrees, so that the cos a term of Eq. (2) must be included in 

 Eq. (5a, b, or c). The oscillation of the coil must produce a change of 

 flux linked by the coil. The movement of a coil along its axis or 

 perpendicular to its axis in a homogeneous field does not produce 

 such a change of flux. 



Figure (3-1)6 shows an example of an oscillating-coil transducer. 

 A frame F of a width b is exposed at one end to a magnetic field with 

 perpendicular component H. The field in the remaining part of the 

 coil is H . (H Q is small compared to H, or it is zero. ) The coil vibrates, 

 driven by a mechanical linkage D, in the direction of the arrow. The 

 rms voltage appearing at the terminals is 



E n = 



2tt 



V2 



fdbri[x{H — H ) 



where d is the oscillation amplitude (measured from the middle 

 position to one side), and/ is the oscillation frequency. The oscilla- 

 tion amplitude should be small compared to the homogeneous zone 

 of the magnetic field. 



