Editing Electrometer (section)

=== Vibrating reed electrometers ===
[[Reed (music)|Vibrating reed]] electrometers use a variable [[capacitor]] formed between a moving electrode (in the form of a vibrating reed) and a fixed input electrode. As the distance between the two electrodes varies, the capacitance also varies and electric charge is forced in and out of the capacitor. The [[alternating current]] signal produced by the flow of this charge is amplified and used as an analogue for the [[Direct current|DC]] voltage applied to the capacitor. The DC input resistance of the electrometer is determined solely by the leakage resistance of the capacitor, and is typically extremely high, (although its AC input [[Electrical impedance|impedance]] is lower).

For convenience of use, the vibrating reed assembly is often attached by a cable to the rest of the electrometer. This allows for a relatively small unit to be located near the charge to be measured while the much larger reed-driver and amplifier unit can be located wherever it is convenient for the operator.<ref>[https://frank.pocnet.net/sheets/009/x/XL7900.pdf XL7900 Vibrating capacitor data sheet]</ref>