456 



SEISMIC METHODS 



[Chap. 9 



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/nterftromtter- 

 Glass 

 PHOes 



Fig. 9-9. Extension measure- 

 ment by interferometer (Gruen- 

 eisen arrangement). 



Another interference method has been 

 applied by Richards^ for testing deforma- 

 tions of rectangular plates. If such plates 

 are subjected to stresses at the ends, they 

 will curve not only in the longitudinal 

 direction but in the transverse as well (see 

 Fig. 9-10) . The upper surface of the plate 

 or rock slab under investigation is polished, 

 and interference patterns, as shown sche- 

 matically in Fig. 9-11, are observed 

 against a fixed plate. 



The simplest of the electrical devices is 

 the wire-resistance type. An L-shaped 

 lever arm magnifies the extension or de- 

 flection of the specimen. Its end acts as 

 the movable arm of a potentiometer which 

 may be placed in any of the well-known 

 bridge circuits to measure resistance or 

 potential changes. An arrangement of 

 this type is used in Zisman's compressi- 

 bility tester (Fig. 9-13).'^ In the con- 

 denser microphone extensometer,** small 

 variations of distance of two condenser 

 plates are converted into changes of ca- 

 pacity ; in the magnetic gauges^" the induc- 

 tance of iron-core solenoids is varied by 



changes in position of an iron armature. 



The solenoids may be energized with A.C. 

 of intermediate fre- 

 quency and may be used 

 in any one of the well- 

 known inductance 

 bridges. Fig. 9-12 shows 

 a Westinghouse extens- 



FiG. 9-10. Longitudinal and 

 transverse curvature of bent 

 plate. 



ometer, which, for slow deformations, may be operated 

 from a 60 cycle current source. 



Fig. 9-11. Pat- 

 terns of interfer- 

 ence fringes of rock 

 slab when bent as 

 in Fig. 9-10. 



6T. C. Richards, Phys. Soc. Proc, 46(1) (246), 70-79 (Jan. 

 1 1933) 



«« W. A. Zisman, Proc. Nat. Acad. Sci., 19, 653, 666, 680 (1933). 



* A detailed discussion of capacitance strain gauges, with 

 literature, is given on pp. 931-932. 



^ Inductance gauges are discussed further on pp. 932-933. 



