930 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



related. It is possible that abrupt displacements give rise to audible vi- 

 brations, and rock bursts of sufficient strength in turn may be responsible 

 for the release of tension elsewhere in the vicinit\^ 



For recording roof subsidence, Landsberg applied a simple recorder 

 consisting of two telescoping steel tubes. The lower of these rested on the 

 mine floor, while the upper was pressed against the roof by a coil spring 

 and connected to the pen of a recorder supported by the base. The dis- 

 placements resulting from mining by retreating pillars may amount to 

 several centimeters in a day's time. 



The sensitivity of mechanical strain gauges may be increased by the 

 addition of an optical lever. Such gauges have been described by Tucker- 

 man.^^ A sensitive optical gauge can be made by combining a Martens 

 lozenge extensometer with an autocollimation telescope (used in magnetom- 

 eters, see Fig. '8-1 8a). The fixed pin of this extensometer rests on one part 

 of the member to be tested and the lozenge, to which a mirror is attached, 

 rests on another part a few inches away. Displacements as small as 

 1/250,000 inch may be detected. Interferometer gauges, while very sensi- 

 tive, are too intricate for field applications. 



Compared with mechanical and optical gauges, electrical devices have 

 the advantage of smaller dimension and possibility of remote indication 

 and recording. These gauges are used chiefly in the testing of convey- 

 ances, railway tracks, bridges, pipe lines, dams, and foundations. They 

 are applicable also in the investigation of ground and roof subsidence and 

 of rock bursts as discussed above. Generally speaking, an electric strain 

 gauge is a device by which an electric current is controlled or modulated 

 according to the relative position of two of its parts. The current modula- 

 tion, in turn, may be accomplished by variations in (1) resistance, (2) 

 capacitance, and (3) inductance. 



A simple resistance gauge is made from a potentiometer whose sliding 

 contact is actuated by the magnification lever of a displacement meter 

 (see Fig. 9-13). Another resistance gauge, known as the telemeter, 

 employs a stack of carbon disks held by a metal frame under an initial 

 pressure of about 180 lb. in~^. Variations in resistance of the stack result 

 from small deformations of the metal frame and are recorded by a Duddell- 

 type oscillograph. The telemeter unit may be used in a water-tight 

 cartridge for sealing into the concrete walls of dams or similar structures 

 whose internal stresses are to be checked periodically. Because of its 



^* Ibid., with bibliography. 



"s L. B. Tuckerman, Am. Soc. Test. Ma*. Proc, 23(11), 602-610 (1923). 



9« B. McCollum and O. S. Peters, U. S. Bur. Stand. Tech. Paper, 17 (No. 247), 

 737-777 (Jan., 1924). O. S. Peters and R. S. Johnson, Am. Soc. Test. Mat. Proc, 

 23(11), 892-901 (1923). O. S. Peters, Am. Soc. Test. Mat. Proc, 27(11), 522-533 

 (1927). 



