608 
38 
to hydrostatic pressure is computed to be 1.078 x 1071! coulombs per pound. If 
the moduli determined by Réntgen are used, a value 1.087 x 10°" coulombs per 
pound is obtained. The agreement with either of these values is thus quite 
satisfactory. 
Quadrant Electrometer 
An alternative method of calibrating the piezoelectric gage utilizes 
the Compton quadrant electrometer.* This instrument possesses much higher 
sensitivity than that required to measure the typical output of a TMB piezo- 
electric gage. By shunting the gage with a total capacitance of about 0.01 
microfarad, as illustrated in Figure 22, a potential difference of about 0.1 
volt was impressed on the quadrants. Although the instrument is capable of 
many refined sensitivity adjustments, it was found possible to get the de- 
sired range of sensitivities merely by varying the needle potential between 
45 and 65 volts. Because of the high capacitance in parallel with the gage, 
the instrument was usually stable and free from excessive drift. However, 
for satisfactory operation the impedance of the electrometer system had to be 
kept high, of the order of 50,000 megohms. Gages of lower impedance could 
not be calibrated with the electrometer. To prevent moisture from collecting 
on the insulation, a small quantity of calcium chloride was kept inside the 
casing of the electrometer as well as inside the box which shielded the 
switches and capacitors. 
Gages were calibrated in the following manner: After the needle 
potential was adjusted for the desired sensitivity, the "floating" quadrants 
were grounded by momentarily closing switch Ky; see Figure 22. Then, with K, 
| to 10,000 ohms 
| megohm 
. 
= 
o > 
= E wo — 
Sve = o= 
c os 
2 ae — 
Q vt 
Standard 
Cell 
zz Shielding Box Quadrant — 
Figure 22 - Electrometer Calibration Circuit with Potentiometer 
The deflections of a Compton quadrant electrometer are calibrated by putting a known voltage 
on the needle with a Leeds and Northrup Type K potentiometer. 
* The electrometer was obtained on loan through the courtesy of Dr. L.E. Curtis of the Radioactivity 
Section, National Bureau of Standards. 
