MEASUREMENT OF PRESSURES 161 



tional Defense Research Committee, at the Underwater Explosives 

 Research Laboratory (Woods Hole Oceanographic Institution, Woods 

 Hole, Mass.). The experimental techniques employed at these and 

 other laboratories employed tourmaline pressure gauges perhaps more 

 extensively than all other kinds put together. As such techniques were 

 used to obtain most of the results on explosion pressures mentioned in 

 this book, they will be described in some detail in this and following 

 sections. 



Of the various types of crystals which might be used for piezoelectric 

 explosion pressure measurements, tourmaline stands virtually alone in 

 its fortunate combination of properties. Mechanically, it is quite 

 rugged and stable and can be so used as to withstand pressures of the 

 order 40,000 Ib./in.^ or more without damage. Although its absolute 

 sensitivity, expressed as electrostatic charge per unit area developed by 

 unit pressure, is much smaller than for Rochelle salt or similar crystals, 

 it is comparable to quartz in this respect and has the great practical 

 advantage over either quartz or Rochelle salt of responding to hydro- 

 static pressure. 



The fundamental piezoelectrically active orientation of tourmaline 

 is a cut perpendicular to the optic axis, charge being developed on faces 

 perpendicular to the axis in response to pressure being applied both on 

 these faces and on faces parallel to the axis. If the ratio of piezoelectric 

 charge Q/A per unit area developed by hydrostatic pressure P is de- 

 noted by K, it is readily shown that the crystalline symmetry of tour- 

 maline requires that K = d^^ -\- '^dsi, where 6^33 and dsi are, in Voigt's 

 notation, the piezoelectric moduli for stresses applied parallel and at 

 right angles to the optic (Z) axis. 



For X-cut quartz crystals, which have been used in pressure measure- 

 ments, the faces of the crystal are perpendicular to the mechanical axis. 

 The charge developed on these faces by unit stress parallel to the axis 

 is represented by dn, but stresses at right angles develop a charge 

 2di3 and it is easily shown that for equal principal stresses (hydrostatic 

 pressure) the total charge, which is the sum of the effects, is zero. 

 Hence if a quartz crystal is to be used, its edges must be protected from 

 applied pressure. In some gauge designs, this has been done by mount- 

 ing a circular quartz disk in a coaxial steel ring intended to relieve pres- 

 sure on the edge of the disk. 



It is well known that Rochelle salt and the niore recently developed 

 ADP (ammonium dihydrogen phosphate) crystals are piezoelectrically 

 active for shearing stress. In Z-cut crystals the charge is developed 

 on faces parallel to the optic axis as a result of shearing stress applied to 

 edges parallel and perpendicular to this axis. As for quartz, the crystal 

 symmetry requires that the response to hydrostatic pressure be zero. 

 As ordinarily used, Rochelle salt or ADP gauges employ crystals cut 



