Sec. 1-2] MECHANICAL IS ITT TRANSDUCERS 59 



part of the core. The voltage is proportional to the variation with 

 time of the acceleration 



da d 3 s 



1t''^dt z 



Electric integration of this voltage furnishes a signal which is pro- 

 portional to the acceleration. Magnetic shielding 8 is used to reduce 

 the effect of stray magnetic fields and of induction caused by the 

 rotation of the transducer in the earth's magnetic field. Theaccelerom- 

 eter shows a transverse sensitivity (response to acceleration in the 

 direction x—x), an effect which can be eliminated by a mechanical 

 construction which permits the transmission of forces from the mass 

 to the core in longitudinal direction y— y only. Figure ( 1-2)4 lc shows 

 a magnetostrictive transducer in which the applied force causes a 

 change of the mutual inductance between the coils P and S. Figure 

 (l-2)4:ld shows a magnetostrictive phonograph pickup containing 

 a magnetostrictive wire W (nickel, 0.02 in. diameter) arranged 

 between the two poles of a magnet. Two pickup coils are wound 

 around the two halves of the torsion wire. Hysteresis is reduced and 

 linear response is obtained by giving the wire an initial torsional 

 stress, i.e., twisting it and attaching it rigidly to the magnetic poles. 

 A stylus displacement will increase the stress on one side of the tor- 

 sion wire and decrease it on the other side by an equal and opposite 

 amount, leading to an increase of magnetic flux in one-half the wire 

 and decreasing it in the other half. The frequency response of this 

 system is substantially flat over a frequency range from 150 to 15,000 

 cps. A practical form of a magnetoelastic pressure cell is described 

 by Merz and Scharwachter. 1 



The response characteristic of magnetoelastic transducers is in 

 general nonlinear. The highest frequency with which the transducer 

 will respond is determined by the mechanical resonance frequency of 

 the magnetoelastic probe and can be as high as several 10,000 cps. 

 However, eddy currents are likely to introduce limitations at fre- 

 quencies higher than 5,000 cps. Hysteresis errors can be large but in 

 practical instruments can be kept at a value in the order of 1 per cent 

 by aging, by prestressing, and by the use of the transducer over small 

 ranges of stress variations. Some authors have observed parasitic 

 magnetic effects which also disappear when the core is aged (sub- 

 jected repeatedly to high stress). 2 Also a long-time instability 



1 L. Merz and H. Scharwachter, Arch. tech. Messen, V 132-15, November, 

 1937. 



2 See Wilde and Eisele, loc. cit. 



