Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 931 



small size, the unit would also be suited for diamond-drill holes issuing 

 from mine drifts, cross cuts, raises, and the like, where it may be left to 

 record variations in roof or wall stresses as mining operations proceed. 

 With a high-speed oscillograph, these elements may also serve to record 

 rock bursts and blasting vibrations. Several elements in suitable geomet- 

 ric arrangement would permit the taking of travel-time records in connec- 

 tion with blasting operations. From such records the velocity of seismic 

 waves and their variation with time may be calculated. This will furnish 

 data on the variation of rock pressure with time (see discussion at end of 

 this section, page 934). 



Single-stack carbon gauges have the disadvantage that their current 

 characteristics are not linear. This may be overcome by mounting two 

 stacks in one frame, with a tongue projecting between them from the 

 frame. The tongue is actuated by a thrust rod free to move through the 

 frame, the end of the rod being connected to that part of the member whose 

 displacement is to be measured. The two stacks are arranged in opposite 

 arms of a Wheatstone bridge containing an oscillograph in the center arm. 

 This arrangement has the advantage of greater sensitivity and of balanced 

 setup, since the resistance of one stack increases while the other decreases 

 when the thrust rod moves in a given direction. The double unit is well 

 suited for clamping to structural members of bridges, foundations, pipe 

 lines, rails, and the like. 



A unique type of resistance strain gauge has been described by R. 

 Gunn.^^" It consists of a vacuum tube with the cathode filament between 

 two anodes that are mounted together on a rod passing outward through 

 a flexible diaphragm. A displacement of the rod decreases the plate re- 

 sistance of the tube on one side and increases it on the other. The 

 plates are connected to a balanced bridge circuit with microammeter. 

 The magnification is of the order of 10,000. 



Strain gauges depending on variations in capacitance to indicate dis- 

 placement are known as ultramicrometers.^^ Various circuits and methods 

 have been proposed for the measurement of nlinute changes in capacity. 

 These are: (1) the Dowling method, using a grid-tuned or plate-tuned 

 circuit with variable capacity in a Hartley, Colpitt, or similar oscillator; 



»«<■ Rev. Sci. Instr., 11(6), 204 (June, 1940). 



"J. J. Dowling, Phil. Mag., 46(27), 81-100 (July, 1923). C. B. Bazzoni, J. 

 Frank. Inst., 202, 35-50 (July, 1926). S. Ekelof, J. Opt. Soc. Amer., 18(4), 337-341 

 (April, 1929). J. Obata, J. Opt. Soc. Amer., 16(6), 419-432 (June, 1928). H. Olken, 

 Instruments, 5(2), 33-36 (Feb., 1932) ; Electronics, 3, 144 (1931). H. Thoma, V.D.I. 

 Zeit., 73, 639 (1929). R. W. Whiddington, Phil. Mag., 40(139), 634-639 (1920). 

 W. W. Loebe and C. Samson, Zeit. Tech. Phys., 9(10), 414-419 (1928). H. Gerdien, 

 Wiss. Veroff. Siemens Konzern, 8, 2 (1929). H. Riegger and R. Boedecker, Wiss. 

 Veroff. Siemens Konzern, 1, 126 (1920). S. Reisch, Zeit. Hochfrequenztech., 38, 

 101 (1931). G. Gustafson, Ann. Phys., Sec. 5, 22(6), 507-512 (Mar. 21, 1935). 



