Sec. 1-2] MECHANICAL IS PUT TRANSDUCERS 51 



frequency) should be at least 10 times that of the highest frequency 

 component to be measured. 



The influence of the ambient temperature upon the transducer 

 performance is small. The effect of a temperature variation upon the 

 magnetic properties of the core is usually negligible. Increased 

 ambient temperature tends to increase the resistance of the primary 

 coil and leads to a shift of the calibration curve (zero shift error) and 

 to a reduction of sensitivity. The above-mentioned feeding of the 

 transducer from a constant-current supply or the insertion of a 

 thermistor with negative resistance-temperature coefficient in the 

 primary circuit reduces the temperature effect. Commercial trans- 

 ducers have been built for operation in a temperature range between 

 — 50 and (intermittent) 290°C. 



The operation of the transducer can be severely affected by the 

 presence of metal masses in its vicinity and by magnetic a-c stray 

 fields. Such fields can cause an unbalance and an excessive zero 

 voltage as well as nonlinearity of the output. The transducer also 

 generates a magnetic stray field which may induce magnetic dis- 

 turbances in nearby magnetic masses or metal parts (by eddy cur- 

 rents) . Such induced fields may have a back effect on the transducer. 

 The difficulty can be reduced, but never completely remedied, by the 

 use of magnetic shields around the transducers. Such shields should 

 have a full-length slot to reduce eddy currents. 



Linear-variable differential transformers frequently exhibit a 

 transverse sensitivity; i.e., they respond to a radial movement of 

 the core. The magnitude of this transverse sensitivity is in general 

 0.1 to 1 per cent of the axial sensitivity. 



The variable differential transformer is a mechanically simple and 

 rugged transducer suitable for displacements between 0.0001 and 

 several inches (some authors indicate 10~ 6 in. as the lower limit) and 

 forces of at least 0. 1 to 0.3 gram. It has a high output level and a low 

 or medium output impedance. The requirements for accessory 

 equipment are moderate where an accuracy of not more than the 

 order of 1 per cent is required, but they increase considerably for 

 higher demands of accuracy. The dynamic response is limited by the 

 frequency of the supply a-c voltage. 1 



A miniature inductive transducer suitable for intracardial-pressure 

 measurements is described by Gauer and Gienapp. 2 The transducer 

 is 12 mm long and has a maximum diameter of 3 mm. It can be used 

 for pressures from —50 to +250 mm Hg. 



1 See Bulletin AA-1A, Schaevitz Engineering Corporation, Camden, N.J. 



2 O. H. Gauer and E. Gienapp, Science, 112, 404 (1950). 



