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An LCR meter measures inductance (L), capacitance (C) and resistance (R). Its origins lie in AC bridge methods developed to measure individual electrical properties; the clearest documented milestone for a commercial instrument combining all three is General Radio’s Type 650-A Impedance Bridge, introduced in 1933, according to engineer Henry P. Hall’s historical account. The instrument’s path from headphones and manual balancing to digital measurement reflects a broader shift toward portability, speed and production testing.
Before the LCR meter: measuring with AC bridges
The combined instrument came after engineers had already developed ways to measure inductance and capacitance with alternating-current bridges. A bridge is a circuit arrangement that can be adjusted until its opposing electrical effects balance. In a measuring instrument, the operator detects that balance, or “null,” and uses the circuit settings to determine an unknown component’s value.
Inductance measurements in 1927
A 1927 Bell System Technical Journal paper by C. R. Burrows described a shielded Owen bridge for measuring inductance and effective resistance. The paper reported that its apparatus measured inductances from 0.1 to 3 henrys with error below 0.1 percent, and 10 henrys with accuracy better than 0.25 percent. It reported effective-resistance accuracy of about 2 percent. These are results for the apparatus in that study, not general performance figures for bridges. Read the 1927 paper.
Capacitance and the relationship to resistance and frequency
A 1928 Bell System Technical Journal paper described adapting a Wien bridge to measure capacitance in terms of resistance and frequency. It noted that the same method could also determine inductance. Such work supplied metrological foundations for later impedance instruments, but it was not evidence that a combined commercial LCR meter already existed. Read the 1928 paper.
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The 1933 General Radio Type 650-A
In a 2004 history of impedance measurement, Henry P. Hall identified the General Radio Type 650-A Impedance Bridge, designed by R. F. Field and introduced in 1933, as “the first instrument to measure R, L and C.” That is a specific historical claim from Hall’s account, not proof of universal priority across every laboratory instrument or country. Hall also notes that his examples emphasize General Radio: he worked there for more than forty years, and the company documented its work especially well. Read Hall’s account.
One instrument, several bridge arrangements
The 650-A reused bridge components in different configurations for distinct measurements: a Wheatstone bridge for DC resistance, a de Sauty bridge for series capacitance, and Maxwell or Hay bridge arrangements for series or parallel inductance. At 1 kHz, it also indicated capacitor dissipation factor (D) and inductor quality factor (Q). “Bridge” can therefore mean either the circuit topology itself or, more loosely, the complete instrument built around bridge measurements.
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How an operator used it
The 650-A was a null-balance instrument. Its 1 kHz AC source was an electromechanical vibrator; headphones served as the AC detector, while a galvanometer detected DC. The operator adjusted the bridge and listened for the null rather than relying on a digital readout. The unit ran on four large No. 6 dry cells. This arrangement belongs to the 650-A specifically; it should not be assumed to describe every early bridge instrument.
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How postwar electronics changed the instrument
As electronics manufacturing grew after the war, makers needed to test passive components more quickly. A bridge could provide useful measurements, but balancing it took time. Hall traces the resulting push toward faster impedance meters and later automatic measurement through successive technologies: vacuum tubes, transistors, operational amplifiers, integrated circuits, microprocessors and computers.
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- 【Note】Perform open/short calibration before measurement. Fully discharge capacitors and inductors. For onboard components, ensure the circuit is powered off. Do not measure live circuits to avoid damage or inaccurate readings
Transistors brought a more portable bridge
Hall describes the 1957 General Radio 1650-A as one of the first instruments to use transistors. It replaced the 650-A’s headphones with an indicating meter; lower power consumption allowed it to use four D cells in a more portable package. Hall reports 1 percent accuracy for the 1650-A, limited by reading its main dial. That figure applies to this model and its dial-reading limitation; the historical account does not provide standardized, like-for-like testing across generations.
Automatic balance reduced operator effort
A further milestone was the General Radio 1680, described by Hall as a digitally balanced automatic capacitance bridge introduced in 1965. Rather than having an operator bring the bridge to balance manually, the instrument automated that task. This addressed a practical issue for component testing: faster measurement could help increase production throughput. Hall’s history discusses the 1680.
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Digital instruments and the bridge’s continuing role
Digital readouts and control changed how operators interact with impedance instruments, but they did not make bridge methods irrelevant. NIST’s Farad and Impedance Metrology work describes digital bridges used for impedance standards and capacitance measurement, as well as a prototype double-balance LCR meter for comparing unlike impedances. In other words, bridge techniques remain part of precision metrology even as instruments use digital control. See NIST’s Farad and Impedance Metrology program.
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IET Labs’ corporate development history lists a “Modern LCR Meter ‘Digibridge’” in 1976 and the release of its 7660 Precision LCR Meter in 2023. These are milestones on IET Labs’ own timeline, not claims that either instrument was the first of its kind across the industry. See IET Labs’ development history.
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- Rotatable LCD Display : Multi angle adjustment for easy reading.There is no need to hold tester while measuring, more convenient operation
- Multi-function : Auto power off; Data Hold function; Low Power Indication; ZERO ADJ for Capacitance
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What changed from the 650-A to later instruments?
| Generation or milestone | Measurement approach | Operator experience and power | Historical qualification |
|---|---|---|---|
| General Radio Type 650-A, introduced 1933 | Manual null-balance instrument with multiple bridge arrangements | Four large No. 6 dry cells; headphones for AC detection | Details and date from Hall’s 2004 account |
| General Radio 1650-A, described as a 1957 successor | Transistorized bridge instrument | Indicating meter replaced headphones; four D cells supported a more portable package | Hall reports 1 percent accuracy, limited by main-dial reading |
| General Radio 1680, introduced 1965 | Digitally balanced automatic capacitance bridge | Automated balance reduced manual adjustment | Milestone from Hall’s 2004 account |
| IET Labs Digibridge, listed for 1976 | Modern LCR meter, as named in IET Labs’ timeline | Not stated in the cited timeline | IET Labs’ corporate-history entry; not an industry-wide priority claim |
| IET Labs 7660 Precision LCR Meter, listed for 2023 | Precision LCR meter | Not stated in the cited timeline | IET Labs’ corporate-history entry; not an industry-wide priority claim |
The history does not provide a standardized comparison of accuracy across these generations, so the figures above should not be read as a numerical ranking. For readers interested in present-day instruments, a modern digital LCR meter is the current product category—not necessarily a recreation of the 650-A’s circuit or operating method.
Further reading and historic equipment
IET Labs says it continues to manufacture and support selected General Radio metrology products and provides historic catalogs, manuals and refurbished inventory. That may be useful for researching or restoring old test equipment; availability will depend on the specific item. Learn about IET Labs’ General Radio support.
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