48 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



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b. Variable Differential Transformer. Figures (1-2)27jP-3 and 

 (1-2)28 show schematic diagrams of the transducer. It consists of 

 three coils, P, 8 l3 and $2, and a core C of magnetic material. An alter- 

 nating current of a frequency varying, in practice, between 60 cps 

 and 20 kc is fed into the coil P; the alternating magnetic field induces 



voltages E x and E 2 in the nearly identi- 

 cal coils S 1 and S 2 . The output signal 

 E = E-l — E 2 ; for complete symmetry, 

 the output signal is zero. A displace- 

 ment of either one of the coils or of the 

 core position causes a magnetic asym- 

 metry and an output signal. 



The core displacement transducer, as 

 illustrated in Fig. (1-2)29, is most fre- 

 quently used. The core consists usually 

 of a nickel-iron alloy and is slotted 

 longitudinally to reduce eddy currents. 

 The transfer characteristic (output voltage versus core displacement) 

 is shown in Fig. (1-2)30; it is linear over a considerable range and 

 flattens out on both ends. The output voltage changes its phase by 

 180° as the core is moved through the zero position. 



The sensitivity of commercial transducers in the linear part of the 

 transfer characteristic is of the order of 0.5 to 3 m V for a displacement 

 of 0.001 in. /volt applied to the primary coil; under standard operat- 

 ing conditions (6.3 volts at 60 cps) the sensitivity is between 3 and 

 Protective shield-^ /-Insulating torm 



Fig. (1-2)28. Linear variable 

 differential transducer, sche- 

 matic diagram. 



— 



- Core - 



Cross 

 section 



Fig. (1-2)29. Linear variable differential transducer, 

 construction (Schaevitz Engineering Corp., Camden, N.J.; 

 by permission). 



20 mV/0.001 in. The sensitivity and the output voltage can be in- 

 creased by increasing the primary voltage, but such a procedure will 

 lead to an increase of the heating of the primary coil proportional to 

 the square of the primary current and thus to a variation of the 

 resistance and, in further sequence, of the output level and of the 

 sensitivity. The effect can be reduced by feeding the primary coil 



