Null detectors are precision electrical measurement instruments historically used to measure minute voltages. These devices are highly sensitive, capable of detecting voltage differences as low as nanovolts, highlighting their importance in technical applications. Null detectors are characterized by an increase in impedance as the measured voltage approaches zero, effectively functioning like an ideal voltmeter with nearly infinite resistance at near-zero voltage levels. This feature allows them to measure voltage without significantly influencing the circuit.
Typically housed in precision calibration laboratories, null detectors were employed in the calibration of industrial electronics, utilizing equipment such as Kelvin–Varley dividers and various bridge measurement circuits. Due to their sophistication and high cost, these instruments were primarily reserved for laboratory use rather than routine industrial applications. They played a crucial role in establishing traceability to Measurement Standards maintained by the National Institute of Standards and Technology (NIST), linking the performance of common electrical measurement devices like voltmeters, ammeters and ohmmeters to these standards.