| 摘要 |
<p>A combined substitution and nulling off technique using an electrometer as a null indicator, a calibrated variable voltage source (d.c) and three switches is used, the ionised chamber plugging into a standard shielded coaxial socket on the panel of the instrument. The switches are (a) the test chamber shorting switch (b) an internal self capacitance shorting switch (c) the electrometer charging change over switch. The voltage at the slider of the potentiometer is set for null reading of the electrometer with both shorting switches open and the voltage measured. With the cell shorting switch still open, switch (b) is closed and switch (c)restored to the position where the electrometer is connected only to the socket. All internal capacitances are thus short circuited and discharged. (b) Is again opened and (c) restored to the position where the variable d.c. is connected to the electrometer, this source again being adjusted for a null and the voltage measured. Finally with (a) left open, the shorting switch (b) is closed, switch (c) remaining as in the second step, the electrometer is again nulled and this third voltage read. Using these three voltages and the known self capacitance of the electrometer, the charge on the ionising chamber plugged into the test set is calculated.</p> |
