932 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



(2) the Gerdien-Thoma resonance circuit; (3) the Whiddington heterodyne 

 method; and (4) capacitance bridges. The simplest of these arrange- 

 ments is probably the Dowling circuit which contains the variable capaci- 

 tance in the grid circuit, the corresponding plate current variation being 

 read on a sensitive meter provided with a bucking circuit. Obata used 

 this method in a seismograph accelerometer and a pressure gauge. The 

 disadvantage of this circuit is its nonlinear characteristic, the sensitivity 

 decreasing in inverse proportion to the condenser spacing. 



In the resonance method a V.T. oscillator is coupled through a condenser 

 to a measuring circuit that is so tuned as to nearly resonate with the 

 former. The measuring circuit contains the variable capacitance, ampli- 

 tude variations being read on a thermocouple-milli voltmeter. Several 

 commercial machines for matched production control have been developed 

 along this line. In Whiddington's heterodyne ultramicrometer two oscil- 

 lators are employed; one contains the variable capacitance (and is therefore 

 variable in frequency), whereas the frequency of the other oscillator is 

 fixed. In Whiddington's setup, the beat frequencj^ is observed directly 

 in a detector circuit by means of a speaker. In a modification by Loebe 

 and Samson, the beat frequencj'', converted into amplitude variation in a 

 nearly resonant circuit, is then amplified and read on a meter. The 

 capacity-bridge ultramicrometer developed by Reisch has the advantage 

 of a linear relation between reading and displacement. It employs a 

 movable plate between two fixed condenser plates, and stray effects are 

 eliminated by a balanced circuit. 



Ultramicrometer capacitance gauges are exceedingly sensitive. It is not 

 difficult to measure changes m length to a millionth of a centimeter. 

 However, there is frequent interference because of stray capacitances, 

 temperature changes, and the Hke. This probably accounts for the fact 

 that these gauges are used chiefly in plants and laboratories and are not 

 so desirable as the resistance and inductance gauges for use in the field. 



An inductance-bridge strain gauge is essentially a Wheatstone bridge 

 with iron core reactors in opposite arms. The iron core coils are provided 

 with armatures, and the gap between them remains fixed in one of the 

 reactors whereas the other changes with the displacement to be measured. 

 In another form of the inductance gauge there is but one armature with 

 two coils on opposite sides, so that a displacement of the armature in- 

 creases one gap and decreases the other.^^ In still another form of this 

 bridge, two balanced armature coils are in one arm of the bridge while 

 two balancing coils, wound on a transformer core, are in the other arm. 



98 A. V. Mershon, Gen. Elec. Rev., 31(10), 526-531 (Oct., 1928); 35(3), 139-144 

 (March, 1932). C. M. Hathaway and E. S. Lee, Meeh. Eng., 69(9), 653-658 (Sept., 

 1937). M. A. Rusher, Am. Ceram. Soc. Bull., 14(11), 365-367 (Nov., 1935). 



