458 



SEISMIC METHODS 



[Chap. 9 



The piston displacement is measured by one of the optical or electrical 

 methods described before; the pressure P is read (Adams)" on a resistance 

 pressure gauge meter immersed in the liquid. Other investigators have 

 used the reduction in length of the specimen for a determination of the 

 cubic compressibility. They measured the pressure of the liquid with a 

 manometer and determined the reduction of length of the specimen by a 

 contact bar whose motion was magnified by optical or electrical means. 

 Amagat carried this rod outside the pressure cylinder and measured its 

 displacement optically; Bridgman magnified its movement by a lever arm 

 acting as the movable arm on a potentiometer; Zisman used the arrange- 

 ment shown in Fig. 9-13 mth the transmission mechanism enclosed in the 

 pressure bomb. If AZ is the reduction of length of the specimen, the 



linear compressibility may be reduced to the 



potentiomettr 



fSprina 



cubic compressibility, since 

 3AZ 



K = 



IF' 



(^21) 



Because of the many variables and correction 

 factors entering into such compressibility 

 tests, it has become general practice to use an 

 iron cylinder of known compressibility as a 

 reference standard. Considerable magnifica- 

 tion (of the order of one million) is required to 

 measure changes of length accurately. 



Bending (transverse) tests: These tests may 

 be divided into two groups. In the first, 

 Young's modulus alone is determined ; in the 

 other, both Young's modulus and Poisson's 

 ratio are obtained. Tests in the first group 

 are made with bending (beam testing) ma- 

 chines. Rock specimens in the form of slabs are clamped on one end 

 or supported on knife edges on both ends. The last procedure is not 

 so favorable as the first because the deflection is only ^t of that ob- 

 served at the free end of a clamped beam. Deflections are mea- 

 sured optically by mirror devices or by condenser-microphone ar- 

 rangements. If / is the moment of inertia of the beam section 



Fig. 9-13. Zisman's resist 

 ance extensometer for com 

 pressibility tests. 



MF. D. Adams and E. G. Coker, Pub. No. 46, Carnegie Inst., 1906; Gerl. Beitr.; 

 31, 31&-321 (1931). L. H. Adams and R. E. Gibson, Proc. Nat. Acad. Sci., U, 276 

 (1926), 16, 713 (1929). L. H. Adams and E. D. Williamson, J. Frank. Inst., 196, 

 475-629 (1923). L. H. Adams, E. D. Williamson, and J. Johnston, J. Am. Chem. 

 Soc, 287, 7-18 (Jan., 1939). 



