Sec. 1-2] 



MECHANICAL INPUT TRANSDUCERS 



63 



in Fig. (1-2)45. Any change of the magnetic flux O passing through 

 the coil causes, by electromagnetic induction, an output voltage e = 

 —n d<t>ldt. The output voltage is proportional to the rate of change 

 of the magnetic field; if the magnetic field is changed in response to a 

 physical displacement, the transducer output is proportional to the 



'act* 



Fig. (1-2)44. Magnesyn trans- 

 ducer, schematic diagram. 



Fig. (1-2)45. Induction- 

 type displacement trans- 

 ducer, principle. 



time derivative of such a displacement, i.e., to the velocity. How- 

 ever, the output can be used directly as a measure for the displace- 

 ment in those cases where the displacement and the velocity are 

 directly related (e.g. , for harmonic oscillations at constant frequency) . 

 In all other cases the voltage e can be fed into an integrating system, 

 and the output from the latter is 

 proportional to the displacement. 

 An advantage of the induction 

 system over the systems described 

 above is that the output voltage 

 is generated in the transducer and 

 no auxiliary voltage source is 

 required. Therefore, the dynamic 

 response of the transducer is not 

 limited by the frequency of an 

 a-c supply voltage or carrier frequency, and a considerable fre- 

 quency range can be encompassed. The variation of the flux can be 

 accomplished in different ways. 



One method to vary the flux developed for ballistocardiographic 

 measurements is illustrated in Fig. ( l-2)46. x The transducer consists 



1 See also T. A. Perls and C. \V. Kissinger, Natl. Bur. Standards Rept. 27:?:}. 

 September, 1953; G. Ising, Ann. Physik, (5) 8, 911 (1931). 



Fig. (1-2)46. Induction-type displace- 

 ment transducer for longitudinal move- 

 ment [from T. A. Perls and E. 

 Bushmann, Rev. Sci. In-str., 22, 475 

 (1951)]. 



