695 
= tBG ke 
and precise methods of impedance measurement., The work of this laboratory 
has involved extensive development and use of piezoelectric pressure gauges, 
in which the charge developed on the crystal is proportional to the pressure, 
As ordinarily used, these gauges are shunted by large eacttancseye which 
must therefore be known if the charge resulting from pressure changes is to 
be determined from measured terminal voltages, 
This laboratory has been fortunate enough to have the use of a wide- 
range resistance=capacitance bridge of considerable accuracy, which has been 
described elsewhere in detail 
The bridge arms and other impedances are checked periodically against 
General Radio type precision fixed condensers, In order to standardize 
levels of measurements, interlaboratory checks of reference capacitors have 
occasionally been madee 
For capcitance measurements in the field, a bridged-T capacitance com= 
parison circuit has been developed. The development of the "Q-step" cali- 
bration method described in Sec.e 6(b) has provided a better means of cali- 
brating piezoelectric gauge circuits in terms of a fixed condenser. 
(a) Capacitance bridge. -- This bridge is used primarily to measure 
capacitances of electrical cables for fault location and to check condensers 
used in the "Q"-calibration procedure [see Sec. 6(b)]. 
The operation of the bridge is based on a substitution method in a 
bridged-T network (see Fig. 26). This network has zero transmission at a 
15/ 
frequency determined by the two balance conditions :—= 
B Calibration and use of piezoelectric gauges for transient pressure 
measurements are discussed in forthcoming OSRD Reports. 
ib/x, S. Cole and H. J. Curtis, Rev. Sci. Inst. 8, 333 (1937). The 
bridge and associated equipment were made available for this work through 
Kenneth S. Cole and The College of Physicians and Surgeons, Columbia Uni- 
versity, to whom this laboratory is greatly indebted, 
a The balance conditions are readily determined from the fact that 
for zero transmission, the sum of the short-circuit transfer impedances 
of the two parallel paths from oscillator to detector must be zero. For 
a discussion of bridged and parallel T-networks, see W. N. Tuttle, Proc, 
TeReEs 28, 23 (1940). 
