58 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



The forces required at the input of the transducer should be such 

 as to cause in the material a stress not exceeding the order of 700 

 kg/cm 2 (10,000 lb/in. 2 ). Depending upon the cross-sectional area of 

 the probe, forces from several grams up to several tons can be 



measured directly. The displace- 

 ment at the input of the transducer 

 is relatively small and will in 

 general be in the order of several 

 microns. The output varies be- 

 tween 10 and 30 per cent of the 

 inductance at zero mechanical 

 load, otherwise hysteresis effects 

 may become excessive. The out- 

 put inductance is in general of the 

 order of milli-henrys up to a 

 fraction of a henry depending upon 

 the number of turns and the phys- 

 ical dimensions of the magnetic 

 circuit. 



50 100 



Mechanical load, 



150 

 kg 



Fig. (1-2)40. Magnetoelastic 

 ducer, transfer characteristic; 

 schematic diagram of transducer. 



200 



trans- 

 insert, 



£. 



(a) 



^^XZ 



I 



y 



(b) 



~M 



(c) 



id) 



Fig. (1-2)41. Different forms of magnetoelastic transducers: (a) magnetoelastic 

 probe P, surrounded by shielding yoke Y; (b) magnetoelastic accelerometer [from 

 H. Wilde and E. Eisele, Z. angew. Phys., 1, 359 (11149)]; (c) mutual-inductance 

 magnetoelastic transducer; (d) magnetoelastic phonograph pickup [from S. E. Rich, 

 Electronics, 19, 197 (June, 1946)]. 



Practical forms of transducers are shown in Fig. (1-2) 4 la to d. 

 The yoke Y surrounding the magnetoelastic probe Pin Fig. (1-2) 4 la 

 increases the sensitivity by increasing the magnetic flux and acts as 

 a shield against external stray magnetic fields. The yoke as well 

 as the core should be laminated to reduce eddy currents. Figure 

 (1-2)416 shows a magnetoelastic accelerometer as described by Wilde 

 and Eisele. If accelerated, the mass M produces stress in the lami- 

 nated core and changes its permeability; the permeability change 

 causes the induction of a voltage in the coil wound around the central 



