Chap. 12] MISCELLANEOUS GEOPHYSICAL IMETHODS 933 



The supply frequency of inductance bridges is usually 60 cycles, but it 

 may be increased to 500 and 2000 cycles for certain applications. In 

 gauges intended for thickness measurements and similar uses in controlled 

 plant production, the indicators are rectifier meters. When time varia- 

 tions of displacements or vibrations are to be recorded, measurements are 

 made with oscilloscopes or oscillographs. 



General Electric Company has developed several commercial models for 

 the inspection of outside and inside tolerances of machined parts, spacing 

 of holes, and the like, and for the automatic control of machining opera- 

 tions. (A balanced armature inductance gauge of similar construction is 

 illustrated in Fig. 9-12). Inductance gauges are suitable for the measure- 

 ment of strains in foundations, bridges, and dams, and for recording 

 stresses, roof subsidence, and wall and fault displacements in underground 

 workings. In application, the fixed and variable air gaps in the reactors 

 are first adjusted to bring the bridge into balance. Then the variable 

 reactor is taken apart, transferred to the member to be tested, and again 

 adjusted for air gap length to obtain bridge balance. The magnification 

 of inductance gauges is of the order of 100,000 and may be combined with 

 trouble-free operation. 



Various seismologists have attempted to measure displacements between 

 points at the earth's surface. Milne^^ determined the motion between two 

 piers 3 feet apart by attaching a thrust rod to one of them and recording 

 the motion of the free end of the rod with respect to the other. This 

 arrangement is therefore similar to the convergence recorder previously 

 described (see page 930) operathig in a horizontal instead of a vertical 

 direction. Milne's arrangement was modified and increased in sensi- 

 tivity by various investigators, such as E. Oddone,^"" R. Takahashi,^"^ and 

 H. Benioff. ' Benioff used two piers 60 feet apart, with a thrust rod con- 

 nected to one of them. The free end of the thrust rod was provided with 

 a balanced armature reluctance transducer. The currents induced by the 

 movement of the piers with respect to each other were proportional to the 

 displacement velocity; hence, the apparatus functioned essentially as a 

 seismograph and not as a strain gauge. However, it could readily be con- 

 verted into a displacement meter l)y an adaptation of the inductance 

 bridge previously described. 



A number of the strain gauges described above, when reproduced on 

 smaller scale, may be applied in experiments with models of proposed 

 structures, tunnels, or underground workings. It is true that much valu- 



" Trans. Seis. Soc. Japan, 12, 63 (1888). 

 '»« Bull. Soc. Seis. Ital., 11, 168 (1900). 

 '" Bull. Earthq. Res. Inst., 12(4), 760-775 (1934). 

 102 Bull. Seis. Soc. Amer., 25(4), 283-309 (Oct., 1935). 



