EFB

An electrostatic force balance is a precision instrument that measures mass by applying an electric force to counteract the gravitational pull on an object. Instead of using a mechanical pan scale or spring, the EFB suspends a sample in an electric field generated between charged plates. When the electric force exactly balances the weight, the mass can be calculated from the applied voltage and the known geometry of the field.

The measurement works through Coulomb force. A charged object placed between two electrodes experiences a vertical electric force proportional to the voltage applied across them. As voltage increases, this force rises until it equals the downward gravitational force on the sample. At equilibrium, no net force acts on the object, and the mass follows directly from the balance equation. This approach eliminates friction and wear that plague mechanical scales, making EFB instruments extremely stable over time.

EFB balances operate across a wide range of masses, from milligrams to several kilograms depending on the plate geometry and maximum voltage. They typically require an inert atmosphere or vacuum to prevent air currents from disturbing the suspended sample. The electrode spacing, voltage accuracy, and temperature stability of the instrument all affect the final uncertainty. High-end EFB systems achieve uncertainties below 1 microgram, making them valuable in pharmaceutical quality control, precious metal assay, and calibration laboratories where extremely small mass differences matter.

The term reflects the physics at its core: electrostatic refers to the stationary electric field, and force balance describes the equilibrium condition being exploited. The method emerged from fundamental electrical metrology and remains comparatively rare in general laboratories, where mechanical or electromagnetic balances are more practical. EFB instruments demand careful handling, stable environmental conditions, and skilled operation, but their repeatability and freedom from mechanical hysteresis make them indispensable for the highest-accuracy mass measurements.