electrometer

An electrometer is an electrical instrument designed to measure electric charge, voltage, or current with extremely high input impedance. By presenting a minimal loading of the circuit under test, electrometers can detect and quantify very small signals that are not measurable by ordinary voltmeters or ammeters. Modern devices are capable of measuring currents from femtoamps to picoamps, charges from tens of picocoulombs, and voltages in modest ranges, with high stability and low noise.

Electronic electrometers typically use a very high-impedance input stage, such as a MOSFET or JFET, followed by a precision amplifier and a transimpedance or capacitive front end. They may operate in voltage, current, or charge integration modes. Some designs employ a capacitor-resistor feedback network to convert input current to a voltage, while others integrate charge on an isolated capacitor and read out the accumulated charge. Many electrometers include guard rings, shielded enclosures, and careful thermal design to minimize leakage and drift.

Types historically include vacuum-tube electrometers and modern solid-state electrometers. The latter dominate today, offering greater stability, lower noise, and higher input impedance, typically in the range of 10^12 to 10^16 ohms. Calibration against known standards is essential, as bias currents, leakage, and dielectric absorption can affect accuracy.

Applications span physics research, radiation dosimetry, electrostatic measurements in materials science, capacitor testing, and semiconductor device characterization. Electrometers are often used with charge-sensitive detectors, ionization chambers, or specialized sensors to quantify tiny electrical quantities without disturbing the measurements.