102 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 1 



is then converted into electric signals by any one of the mechanical 

 displacement transducers described above. Since these systems of 

 conversion of pressure into electric signals represent merely combi- 

 nations of elements, they are omitted. A thermoelectrical pressure 

 transducer containing a boiling liquid is described by Goodyear. 1 The 

 boiling temperature of the liquid varies with the pressure and is 

 measured by a thermistor. 



1-51. Resistive Pressure Transducer 



If a wire is subjected to an increase of external pressure exerted 

 from all directions, Fig. (1-5)1, its electrical resistance will change. 



The resistance will usually decrease with pressure, 

 although in some metals it increases, and in 

 cesium it decreases first and increases at higher 

 pressures. The effect has been extensively stud- 

 ied by Bridgman 2 and is caused by a distortion 

 of the crystal lattice in the metal brought about 

 by the external pressure. 3 



For many metals and over small ranges oi 

 pressure variations, the resistance changes lin- 

 Fig. (1-5)1. Bridg- early with a variation of pressure Ap. 



man-type pressure 



transducer, schema- R — _R Q (1 _|_ 5 Ap) (1) 



tic diagram. 



where R is the resistance at 1 atm pressure and 

 b the pressure coefficient of resistance. Average values of the 

 pressure coefficient of resistance for various metals are given in 

 Table 6. 



The coefficient b is largest for the alkali metals and for bismuth 

 and antimony, but transducers made from such materials are not 

 practical, and mercury and manganin are preferred. Mercury has 

 the advantage that it can be produced with uniform purity, but it 

 undergoes a phase transition near room temperature in the pressure 

 range between 6,500 and 7,600 kg/cm 2 ; in this stage the pressure 

 coefficient of resistance changes slightly (see Table 6). 



The pressure coefficient of resistance does not change much with 



1 R. S. Goodyear, Elec. Mfg., p. 90, October, 1956. 



2 P. W. Bridgman, "The Physics of High Pressure," The Macmillan Com- 

 pany, New York, 1931. 



3 N. H. Frank, Phys. Rev., 47, 282 (1935), and J. C. Slater, "Introduction to 

 Chemical Physics," chap. 29, McGraw-Hill Book Company, Inc., New York, 

 1939. 



