Electrometer Circuit
An Electrometer is an amplifier with an extremely high input impedance. Thus, when an electrometer is shunted across a capacitance, its input impedance will not try to discharge the capacitor.
The high input impedance does not bleed the charge off the capacitor at it measures its change voltage, so the voltage across the sensor capacitor is a function of the parameter being measured or, in the case of a proximity or presence sensor, the presence or absence of an object or intruder.