384 



EXPLORATION GEOPHYSICS 



Fig. 229a. — Field procedure for reading portable meters. (A) carrying 

 case and cover; (B) gravity meter; (C) tripod. (Courtesy of International 

 Geophysics Company.) 



One method is illustrated by the Boliden gravimeter previously described. Because 

 the total capacity is usually small, measurements are generally made at high frequency, 

 which is most conveniently supplied by a small vacuum tube oscillator. In general, 

 electric gauging methods utilize bridge-type circuits. The circuit is simple, comprising 

 the conventional four-arm alternating-current bridge. In one arm is placed the capacity 

 or the inductance whose value is changed by the movement of the gravity-meter system. 



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Fig. 230. — Electric gauge, bridge-type gravimeter. 1, 

 mass; 2, suspension lever; 3, spring hinge; 4, suspension 

 spring; 5, spring lever; 6, adjustment screw; 7, adjustment 

 scale; 8, adjustment knob; 9, bimetallic temperature com- 

 pensation lever; 10 and 10', electric gauge heads, differen- 

 tial type; 11 and 11', armatures; 12 and 12', reactor coils; 

 13, alternating current vacuum tube voltmeter; 14, cast 

 aluminum case; 15, thermal insulation; 17, polished dur- 

 aluminum case; 18. leads to alternating current supply. 

 (Courtesy International Geophysics, Inc.) 



