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39 
in position A, the pressure on the crystal was released. The corresponding 
ballistic deflection was read on a scale about 1 meter from the electrometer. 
This was repeated for various pressures. 
Voltage calibration of the scale was carried out as follows: First 
K, was momentarily closed. Then K, was thrown to position B, charging up a 
mica capacitor C, to a potential difference which was precisely determined 
with a Leeds and Northrup Type K potentiometer. By reversing K, the known 
voltage was impressed upon Co and the quadrants. Deflections of the needle 
were observed for a suitable range of voltages. 
Voltages AV were plotted against corresponding pressure changes AP 
and a "best line" was drawn. Usually the experimental points were so nearly 
collinear that the slope AV/AP could be determined without recourse to the 
method of least squares. The piezoelectric constant KA of the gage is given 
by C(AV/AP), where C is the total capacitance in parallel with the gage. C 
was measured with a Schering 
capacitance-bridge circuit 
manufactured by the General 
Radio Company. 
The results obtained 
with this method of calibration 
are described at the end of the 
next section. 
Microcoulometer 
A third calibration 
technique makes use of an im- 
pedance coupler, Figure 23, 
with a large time constant, 
together with a voltmeter con- 
sisting of a moving-coil gal- 
vanometer* in series with a Figure 23 - Piezoelectric Gage Impedance 
suitable resistance. This voup Ler ee pare? Jere oe ay 
circuit has been called a mi- 
crocoulometer. It has an in- 
put impedance of 30,000 megohms and an input capacitance of 10° farad. Hence 
its time constant, about 200 seconds, is long in comparison with the time of 
deflection of the galvanometer coil, which is about 2 seconds. By the switch 
K), Figure 24, either the piezoelectric crystal or a potentiometer may be 
* the galvanometer employed is manufactured by the General Electric Company under Serial Number 32C. 
