610 
HO 
Potentiometer 
Crystal 
Test Set 
Figure 24 - Calibration Circuit Employing 
the Microcoulometer 
The circuit in Figure 23 is represented by the box 
labeled "Impedance Coupler." The "input" and "output" 
of Figure 23 correspond to connections K, and K,, 
respectively. The galvanometer G together with the 
series resistance R constitute the voltmeter. 
Impedance 
Coupler 
(Microcoulometer) 
scale deflections of the galvanometer for each release of pressure. 
connected to the input of the 
impedance coupler. With K, the 
output can be fed either into the 
galvanometer or into a Simpson 
test set. 
The following calibra- 
tion procedure is used: First 
the gage is momentarily shorted 
by the switch K,. Then the var- 
fable resistor in the impedance 
coupler is adjusted until the 
output potential is zero. For 
preliminary adjustment of the 
output, the test set is a conven- 
tent indicator. In the interest 
of uniformly high accuracy, it 
is desirable to get nearly full- 
This is 
achieved by varying the series resistance R, which controls the galvanometer's 
voltage sensitivity. 
To translate galvanometer deflections into volts, the potentiometer 
is employed in much the same manner as that described for the electrometer. 
Several voltages are applied to the microcoulometer for each value of R pre- 
viously used, and the corresponding galvanometer deflections are noted. With 
these data it is possible to plot voltage against pressure change and as be- 
fore, KA is determined from C and the slope AV/AP of the curve. 
The values of the calibration constant KA obtained with the elec- 
trometer and the microcoulometer are between 7 and 13 per cent lower than 
those determined with the oscillographic recording technique. 
The oscillo- 
graphic values are judged to be correct within the experimental error, inas- 
much as they have been independently substantiated by workers in two other 
laboratories. 
that found by both Voigt and R6ntgen. 
Moreover, they lead to a value of K which agree closely with 
Both the electrometer and microcoulome- 
ter methods offer considerable difficulty because of the high input impedance 
which must be maintained for successful operation; the former is especially 
troublesome as it requires an impedance of 50,000 megohms. 
The discrepancy 
of 7 to 13 per cent has recently been accounted for (25). 
Program for Further Piezoelectric Calibration Studies 
Several important questions concerning piezoelectric calibrations 
remain to be answered. 
Some observers have found an indication of a differ- 
ence between the KA value determined with a "bare" crystal, i.e., before the 
