Sec. 1-2] MECHANICAL INPUT TRANSDUCERS 79 



B, C, and D. A displacement of the point A in t he direction of the 

 arrow will cause a twist ing motion of the element and will give rise to 

 an output voltage. Another application is shown in Fig. (l-2)62c, 

 where a torsional motion of the drive shaft D causes a twisting 

 motion of the bimorph and induces an output signal. A third appli- 

 cation, illustrated in Fig. (1-2)62^, shows the conversion of a move- 

 ment into the twisting of the piezoelectric element and, in further 

 sequence, into an electric output signal. 



The double curvature in bimorphs, both of the bender and of the 

 twister type, requires special precaution in mounting the element. 

 An ideal mounting should permit the desired bending or twisting 

 motion without interfering with the normal flexure of the element. 

 This is generally difficult; a practical approach is to hold bimorphs 

 cemented or clamped between rubber pads. 



The mechanical resonance frequency of bmiorphs is considerably 

 lower than that of the single or stacked piezoelectric elements. 



materials. Twenty of the thirty-two crystallographic classes 

 exhibit piezoelectric properties, but only a few materials are practical 

 for piezoelectric input transducers, primarily quartz, rochelle salt, 

 ammonium dihydrogen phosphate (ADP), and ceramics made with 

 barium titanate. Materials used less often are tourmaline, ethylene 

 diamine tartrate, dipotassium tartrate, potassium dihydrogen phos- 

 phate, and lithium sulfate. 



Quartz (Si0 2 ) is mechanically and thermally the most stable among 

 the piezoelectric materials. Its internal electric losses are small, its 

 volume resistivity is higher than 10 14 ohm-cm. Because of its rela- 

 tively small piezoelectric effects, its application is restricted to such 

 uses where high tensile strength, high mechanical stability, or opera- 

 tion at elevated temperature is essential (the material can be operated 

 safely up to 550°C) . Quartz plates are applicable primarily for thick- 

 ness and transverse compression-expansion operation, Fig. (l-2)59a 

 and b. 



For the technical preparation and testing of quartz elements, see R. A. 

 Heising, "Quartz Crystals for Electrical Circuits," D. Van Xostrand Company, 

 Inc., Princeton, N.J., 1946. 



Rochelle salt (NaKC 4 H 4 6 -4H 2 0) is stable at room temperature 

 between 35 and 85 per cent relative humidity. At higher humidity it 

 will deliquesce; at lower humidity it will dehydrate. Coating with 

 wax is recommended to retard the humidity effects. At a tempera- 

 ture above 55°C the crystal will disintegrate. 



The dielectric constant in the direction of the X axis of the crystal 



