160 MEASUREMENT OF PRESSURES 



response to applied mechanical stress. The effect is exhibited only by 

 crystals of fairly low degree of symmetry, the practically most useful 

 being quartz, tourmaline, and Rochelle salt. The first application of 

 piezoelectric methods to measurement of underwater explosion pres- 

 sures is apparently that of Keys (58) in 1921, as a result of a suggestion 

 by Sir J. J. Thomson that the naturally occurring mineral tourmaline 

 would be a suitable material. At about the same time, Langevin was 

 developing the use of quartz crystals for underwater signalling and 

 Nicolson had made pioneer investigations of Rochelle salt crystals as 

 electromechanical transducers. The experiments of Keys demon- 

 strated the feasibility of the method and tourmaline gauges have 

 been extensively used by the Admiralty Research Laboratory in 

 England from 1919 to the present.'' Up to the outbreak of World 

 War II these investigations were virtually the only sustained program 

 of underwater explosion pressure measurements. 



The original form of equipment used in these experiments was very 

 simple. The gauge consisted of a mosaic of tourmaline crystals at- 

 tached to a steel plate, which formed the low potential (grounded) 

 electrode, a foil electrode connected the crystal faces of opposite polar- 

 ity, and the whole assembly was mounted in a waterproof case. The 

 total crystal area w^as some 30 square inches and the linear dimensions 

 of the gauges were of the order 5-8 inches. With these large gauges, 

 sufficient signal was developed by pressures of the order 200-2,000 

 lb. /in. 2 to permit direct connection of the gauge through cables several 

 hundred feet long to the electrostatic deflecting plates of a cathode ray 

 oscillograph. In the earlier experiments a time scale was provided by 

 horizontal deflection of the trace from a condenser discharge circuit 

 suitably synchronized with the arrival of the pressure wave at the gauge. 

 The resulting deflection of the electron beam was recorded photo- 

 graphically on a film placed in the evacuated cathode ray tube cham- 

 ber. The deflection sensitivity and time scales were calibrated by 

 impressing a transient oscillation of known amplitude and frequency. 

 Although this original technique has been refined and extended greatly 

 as a result of advances in electronics, it contains virtually all the essen- 

 tial components found in later developments. 



At the beginning of World War II, it became evident that piezoelec- 

 tric or similar techniques were essential to obtain comparisons of mili- 

 tary explosives, estimates of their effectiveness in various circumstances, 

 and to make possible comprehensive studies of underwater explosion 

 pressures. With these and similar objectives in mind, experimental 

 programs were started at a number of laboratories in this country, the 

 most extensive being at the Taylor Model Basin (U. S. Navy, Bureau 

 of Ships) and, under contracts of Division 8 and Division 2 of the Na- 



^ Many of these investigations have been reported by Wood (123). 



