Null detector
Instrument to measure minute voltage
A null detector produced by the Fluke Corporation

A null detectors is an electrical measurement instrument used to measure minute voltages. High precision null detectors can resolve voltage differences in the nanovolt range. Null detectors are typically designed to have high input impedance to minimize loading the circuit under test.

Null detectors have historically used in calibration laboratories in conjunction with Kelvin–Varley dividers and bridge measurement circuits. Because of their sensitivity and specialized operating range, null detectors were more commonly used in laboratory than in routine field measurements. They were used as part of measurement chains that established traceability to national electrical standards, such as those maintained by National Institute of Standards and Technology (NIST).

History of Null Detectors

The development of null detectors is closely tied to the use of balance and comparison methods in electrical measurement, where the detector serves only to indicate the presence of an imbalance rather than to measure an absolute quantity.[1]

Key stages

  1. Galvanometers as null detectors: Early galvanometers, were commonly used as indicators of current or imbalance in circuits. In early balance and comparison arrangements, they were used to detect when two points reached equal potential, without providing calibrated measurements.
  2. Bridge circuits: The adoption of bridge circuits, such as the Wheatstone bridge, clarified the role of the detector as a balance indicator.[1] Measurement accuracy depended primarily on achieving a balance, while sensitivity was limited by the galvanometer's electromechanical design.
  3. Vacuum tube null detectors: Electronic amplification enabled galvanometers to be replaced by vacuum-tube-based null detectors with higher sensitivity and without mechanical limitations of galvanometers. Null detectors utilizing vacuum tubes amplifiers were produced commercially and used in conjunction with bridge measurement circuits.[2][3][4]
  4. Solid state null detectors: The introduction of transistor-based amplifiers enabled the development of electronic null detectors with substantially improved sensitivity. By the mid-20th century, solid-state null detectors were produced commercially for laboratory and calibration use, combining electronic amplification with stable DC performance.[5][6]

Applications

Null detectors are designed to resolve very small voltage differences, rather than to measure voltage accurately over a wide range. In null measurements, the detector sense only the small residual difference between an adjustable reference and the quantity under test, so measurement accuracy depends on achieving balance rather than on the detector's absolute calibration.[7] In this mode of operation, the detector is used to indicate balance, and the node potential difference is typically reduced to microvolt-level or below by adjusment of external circuits. Under these conditions, the current drawn by the detector remains small even if its input resistance is modest compared with that of a general purpose voltmeter. For example, solid state null detectors typically has an input impedance of 1 MΩ on its most sensitive ranges,[5][6] whereas digital multimeters typically has an input resistance of >10 GΩ on similar ranges.[8][9]

Null detectors are commonly used with known reference components, such as standard resistors or capacitors, to determine unknown values using bridge circuits including the Kelvin-Varley divider, the Wheatstone bridge, and related configurations. Variants such as the Kelvin Double Bridge allow accurate measurement of low resistance by reducing the influence of lead and contact resistance.

Although many modern digital instruments can perform similar measurements directly, bridge methods employing null detectors continue to be used in electrical metrology for high-accuracy comparison of voltages, resistances, and other electrical quantities.[10]

  • Conrad Hoffman, “Building a Mini Metrology Lab” - hobbyist demonstrations of classical resistance and voltage measurement techniques
  • Paul’s DIY Blogs, “Null Detector Amplifier” - DIY construction of a null detector for educational and experimental use

References

  1. ^ a b "A History of Impedance Measurement" (PDF). General Radio Company / IET Labs. Retrieved 2026-01-01.
  2. ^ "Type 1231-B Amplifier and Null Detector Manual" (PDF). IET Labs / General Radio. Retrieved 2025-01-01.
  3. ^ "General Radio 1231-B Amplifier and Null Detector". Radiomuseum.org. Retrieved 2026-01-03.
  4. ^ Turner, Rufus P. (1967). Bridges and Other Null Devices. Howard W. Sams & Co., Inc.; The Bobbs-Merrill Co., Inc. Library of Congress Catalog Card No. 67-2316.
  5. ^ a b Tektronix (2010). Model 155 Null Detector–Microvoltmeter Instruction Manual (PDF). Manual No. 29031 Rev. D. Tektronix. Retrieved 2025-01-01.
  6. ^ a b "Fluke 845A/845AB High Impedance Voltmeter/Null Detector Instruction Manual" (PDF). Fluke Corporation. Retrieved 2026-01-03.
  7. ^ Calibration: Philosophy in Practice. Fluke Corporation. 1994. ISBN 0963865005.
  8. ^ "Keysight 34461A 6½-Digit Digital Multimeter Data Sheet" (PDF). Keysight Technologies. Retrieved 2026-01-01.
  9. ^ "8588A Reference Multimeter Specifications" (PDF). Fluke Calibration. Retrieved 2026-01-03.
  10. ^ "Final Report of EUROMET.EM-K8: Comparison of DC Voltage Ratio". Bureau International des Poids et Mesures. Retrieved 2026-01-01.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Null_detector&oldid=1331066082"