Sec. 1-2] MECHANICAL INPUT TRASS I WCERS 65 



built with very small moments of inertia and can be used for a fre- 

 quency range extending from a few cycles per second to more than 

 10 kc. 1 



The so-called variable-reluctance phonograph-pickup system is a 

 further example of an induction-type transducer. It is illustrated in 

 Fig. (1-2) 49 and contains a permanent magnet with pole pieces N and 

 S and an iron armature A moving around 

 a pivot. Two rubber blocks B impart a Coil-. 

 restoring force to the armature and prevent 

 it from sticking to the pole faces. As the 

 armature moves, the magnetic flux through 

 it varies and the flux variation induces in 

 the (fixed) coil a voltage 



_ nMv Fig. (1-2)49. Reluctance- 



o J2 variation displacement trans- 



ducer [from S.Kelly, J . Audio 



where n is the number of turns of the coil, Eng. Soc, 2, 163 (1954); by 

 J/ the magnetomotive force in the gap, vthe permission]. 

 velocity of the armature tip in the gap, and 



d the gap width between the armature and the pole. The maximum 

 displacement for which such pickup systems can be used is of the 

 order of 0.1 mm; larger displacements cause a nonlinear distortion. 

 The effective mass of the moving part can be reduced to below 5 mg, 

 and the compliance (displacement per applied force) can be as high 

 as 2 x 10~ 6 cm/dyne. Although the armature has a resonance (in 

 some cases around 3 to 4 kc), its effect can be practically eliminated 

 by appropriate damping, and a fairly linear frequency response (al- 

 though not flat, its output increases with frequency) can be obtained 

 in a range from 40 to 15,000 cps. The output impedance depends on 

 the coil construction; for high-impedance pickups, coils of about 

 5,000 turns are used, having a d-c resistance of 2,000 ohms and an 

 inductance of about 1 henry. For a movement with a velocity of 

 about 10 cm/sec at a frequency of 1 kc the output is of the order of 

 lOmV. 



Induction systems are used frequently in displacement measure- 

 ments of rotating systems in which a rotating permanent magnet 

 induces a voltage in a stationary coil, so that no slip rings are 

 required. A system for the measurement of torque is described by 

 Oesterlin. 2 Two rotating magnets induce a-c voltages in two-coil 



1 S. Kelly, J. Audio Eng. Soc, 2 (3), 169 (1954); see also E.M. Villchur, Audio, 

 February, 1956, p. 40. 



2 W. Oesterlin, Arch. tech. Messen, V 61-3, July, 1952. 



