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Avometer

The Avometer, commonly known as the Avo, is a brand of analog that measures electrical current (amperes), voltage (volts), and resistance (ohms) in a single portable instrument, marking it as the world's first true . Invented in 1920 by British Post Office engineer Donald Macadie to address the inconvenience of carrying separate meters for different electrical parameters, it was initially designed for (DC) measurements only. Commercial production began in 1923 through the Automatic Coil Winder and Electrical Equipment Company (ACWEECO), founded in by R. H. Rawlings to exploit Macadie's , with the first model launched that year. The device quickly became a standard tool for electricians, engineers, and radio technicians, featuring a moving-coil mechanism for precise analog readings and evolving through models that added (AC) capabilities in 1933. Key models include the Model 7 (introduced 1936), which offered 1,000 ohms per volt sensitivity and protective features like an automatic overload cut-out, and the Model 8 (launched 1951), renowned for its higher 20,000 ohms per volt sensitivity tailored to the growing ; the latter remained in production until 2008, by which time AVO had manufactured over a million multimeters overall since 1923. The company underwent several name changes, becoming Avo Ltd. in 1957 and eventually part of Megger Group Ltd. in 1987, reflecting its enduring legacy in electrical testing equipment. Widely regarded as a icon of 20th-century , the Avometer influenced design for decades and was essential for fault diagnosis in applications ranging from to electrical maintenance, though it has largely been supplanted by modern alternatives.

History

Founding of AVO and Early Instruments (1920s–1930s)

The Automatic Coil Winder and Electrical Equipment Company (AVO) was founded on 23 May 1923 in to manufacture inventions including the developed by MacAdie, a senior in the General Office's Factories Department, as a private company specializing in electrical equipment for coil winding and testing. MacAdie, frustrated with the need to carry multiple separate instruments for , developed inventions including an automatic coil winder and a multifunctional meter to streamline testing processes for and radio applications. The company was established specifically to manufacture these devices, with initial production based at Winder House on Douglas Street in . In the same year, MacAdie invented the first Avometer, a DC-only portable instrument that combined measurements of amperes, volts, and ohms in a single unit, deriving its name from these functions (AVO). Patented under British Patent No. 200977 and U.S. Patent No. 1,593,024 (filed 14 May 1923), the design featured simplified switching mechanisms that allowed most measurements using just two input terminals, reducing complexity compared to prior separate meters. Initial sales targeted radio engineers and Post Office technicians in the UK, where the instrument's portability and multifunctionality addressed the growing demand for efficient testing in the expanding broadcasting and telephony sectors. By 1933, AVO introduced the Universal Avometer, an advanced model that extended capabilities to both AC and DC measurements across 36 ranges, incorporating a rectifier for AC functionality and enhancing overall versatility for professional use. The design adopted a Bakelite front panel and casing, which improved durability and portability over earlier metal versions, making it more suitable for field work by electrical and radio professionals. These innovations solidified AVO's early dominance in the UK market, where the Avometer series became the standard tool for radio servicing and electrical testing, prized for its reliability and ease of operation in an era of rapid technological advancement.

Wartime and Post-War Developments (1940s–1960s)

In 1939, the Automatic Coil Winder and Electrical Equipment Co. (AVO) replaced its 1933 Universal Avometer with the Model 40, which featured updated measurement ranges and incorporated shared patented design elements, such as a mechanically operated cut-out mechanism linked to the meter movement, enhancing its robustness for demanding applications. During , the Model 40 and related variants, including Admiralty Patterns 47A and 48A, saw extensive production for the , where they were used by engineers for maintaining systems, circuits, and other military electronics, supporting Allied operations through reliable field testing. Following the , AVO released an updated version of its High Avometer, designated Model 7, in 1947, which achieved a sensitivity of 20,000 ohms per volt on ranges to enable accurate high-resistance measurements, positioning it as a preferred tool for professional electricians and power engineers in reconstruction. This model incorporated wartime-developed improvements, including 50 measurement ranges for voltage, , and , along with self-contained operation via internal batteries. The Model 8 was launched in 1951 as AVO's flagship general-purpose for the civilian market, featuring dual rotary switches that provided access to 50 ranges and improved measurement capabilities with a DC sensitivity of 20,000 ohms per volt. It evolved through several marks, with the introduced in 1956 adding a 500 V range and terminals for enhanced usability, followed by the Mark III in 1963, which included protection for resistance ranges to safeguard against overloads. In the , AVO introduced the Model 9, primarily as an export-oriented variant of the Model 8, incorporating international symbols on its switches and refined scales for broader global compatibility, while maintaining high sensitivity and extending frequency response up to 10 kHz for applications. During the and , AVO experienced significant growth, renaming to AVO Ltd. in , expanding to 700 employees by 1961, and being acquired by Metal Industries Group in 1959, which facilitated increased production and exports to countries, establishing the Avometer—particularly Models 7 and 8—as a benchmark for analog reliability and precision in professional settings worldwide.

Modern Era and Acquisitions (1970s–Present)

In the 1970s, AVO expanded its product line with specialized variants of existing models, including the Avometer Model 9 Mk. IV, introduced in 1969 and refined for high-sensitivity applications in voltage, current, and resistance measurements, particularly suited for technicians requiring 20,000 ohms per volt . This period also saw the launch of the "70 series" pocket-sized analog multimeters, such as the Model 72, featuring compact 3-inch scales and 20,000 ohms per volt , aimed at field engineers needing portable tools for on-site testing. These developments reflected AVO's adaptation to growing demands in the , with production consolidating at new facilities in , , following acquisitions of related firms like Foster Transformers in the 1970s and Evershed & Vignoles (Megger) in 1968. Corporate changes accelerated in the late and , beginning with a 1987 that renamed the company Megger Instruments Ltd., followed by its acquisition by the Thyssen Bornemisza Group in 1991, which merged it with Biddle Instruments and Multi-Amp to form AVO International (later AVO Megger Instruments). In 2002, the company rebranded fully as Megger Group Ltd., shifting production to modern facilities in the , US ( and ), and emphasizing global compliance with safety standards like IEC 61010 for electrical test equipment. This era marked the transition to digital technologies, with the release of digital Avometers in the and 2000s, including models like the DA 212, which combined traditional analog aesthetics with LCD displays, auto-ranging, and true capabilities for measurements up to 1000 V. The classic analog Model 8, in production since 1951, saw its final variants like the Mk. VII (1996) before discontinuation in October 2008 after 57 years, with the last unit serialized and awarded in a 2010 competition. Under Megger ownership as of 2025, the Avometer brand persists on a range of and hybrid tools for electrical maintenance, including the AVO850 series true multimeters with analog graphs for visual continuity and phase rotation testing (with the AVO830 having been discontinued), alongside insulation testers like the MIT200 series that incorporate Avometer heritage in high-voltage diagnostics up to 1 kV. These evolutions ensure the brand's relevance in modern electrical testing, blending legacy reliability with precision for professionals in power distribution and industrial maintenance.

Technical Features

Design and Construction

Avometer instruments were renowned for their robust physical construction, designed to withstand demanding professional use while maintaining accuracy and safety. Classic models, such as the Model 8, utilized or phenolic resin casings that offered excellent electrical insulation and mechanical durability against impacts and environmental hazards. These casings typically measured around 190 mm in height, 170 mm in width, and 100 mm in depth, balancing portability with a stable base for benchtop operation. The choice of ensured resistance to moisture and chemicals common in electrical testing environments. The core of the instrument's mechanical design centered on the meter movement, which employed a pivot-and-jewel system with hairsprings to provide precise control and minimize friction. The moving coil, wound on a frame, pivoted on polished steel knife-edges resting in conical jewels, enabling smooth deflection while reducing errors through an anti-parallax mirror above the scale. This configuration offered good resistance for field use. Overload protection was integrated via fuses in the input circuits, safeguarding the movement from excessive currents or voltages. User-friendly engineering was evident in the dual rotary selector switches, which allowed independent selection of measurement function and range, streamlining operation without complex wiring changes. The white dial face featured clearly marked scales for intuitive reading, with the two-terminal input design further simplifying connections by requiring only positive and common probes for all functions, thereby avoiding separate ground terminals and minimizing setup errors. Internal wiring relied on point-to-point connections with high-quality resistors and capacitors, often hand-assembled for reliability. Over time, construction evolved from the metal-framed aluminum cases of early pre-war models, which provided structural rigidity but added weight, to plastic-insulated versions that improved portability, reduced manufacturing costs, and enhanced user safety through better isolation from live circuits. This shift to materials also facilitated easier integration of batteries and fuses in accessible compartments, maintaining the instrument's hallmark ruggedness while adapting to modern fieldwork needs.

Measurement Capabilities

Avometers provided robust measurement capabilities for professional electrical testing, encompassing and voltage measurements up to 1000 V, and current up to 10 A, and up to 20 MΩ in later models, with a of 20,000 Ω/ across all voltage ranges in high-sensitivity models such as the Model 8. These instruments featured input impedances calculated as Z = × range for voltage measurements, ensuring minimal loading on circuits; for example, the 20 kΩ/ yielded an impedance of 20 kΩ per volt of range selected. Accuracy was typically ±1% of full-scale deflection for voltage and current, and ±2% for measurements at 50 Hz on most ranges. For AC measurements, post-war models offered a frequency response within ±3% error from 15 Hz to 15 kHz on voltage ranges from 10 V to 300 V, suitable for audio and radio applications. Specialized scales included decibel markings from –10 to +55 dB overlaid on AC voltage ranges, facilitating signal level assessments in audio and RF work. Overload protection was achieved through an electromechanical cut-out mechanism, supplemented by fuses rated at 1 A and 10 A, the meter movement from excessive currents or voltages. adhered to British Standard BS 89 for indicating instruments, using Weston standard cells for voltage references and precision standard resistors for ohmic measurements, with traceability to the National Physical Laboratory for metrological accuracy. Despite their precision, Avometers lacked auto-ranging, necessitating manual range selection via a rotary switch, which could introduce errors if mischosen. They were also limited for low-level signals below 1 without external amplification, as the lowest DC voltage range started at 100 full scale, potentially loading sensitive circuits.

Principal Models

Pre-1940 Models

The original Avometer, designated Model 1 and introduced in 1923 by the Automatic Coil Winder and Electrical Equipment Co., represented the first portable designed specifically for the emerging radio industry. This -only instrument featured 10 ranges, including DC voltages of 12 , 120 , and 600 ; DC currents of 120 , 1.2 A, and 12 A; and resistance measurements up to 10 kΩ using a central zero of 225 Ω. Its sensitivity was 1,000 Ω/, achieved with a 12 full-scale deflection current, making it suitable for basic electrical testing by hobbyists and technicians. The meter employed a patented moving-coil mechanism developed by Donald Macadie, emphasizing rugged construction for practical use. The Universal Avometer, launched in 1933 and refined as the 36-range Model 5 in 1934, introduced measurement capabilities, expanding the device's versatility for both DC and applications. It provided 36 ranges, encompassing DC voltages from 0.12 V to 1,200 V, DC currents up to 12 A, voltages up to 1,200 V (with an added 480 V range via a ), and up to 1 MΩ requiring external voltage for higher values. DC sensitivity reached 2,000 Ω/V with a 6 mA full-scale deflection, which could be doubled to 3 mA using the innovative "divide by 2" for enhanced precision on lower ranges; sensitivity was 1,000 Ω/V. Weighing approximately 2.95 kg including leads, the instrument was available with an optional leather carrying case for portability. Early models like this used a in the meter movement for reliable operation in varied conditions. Introduced in December 1939, the Model 40 built on prior designs with 40 ranges, offering improved performance for industrial and general electrical work while maintaining compatibility with pre-war standards. Ranges included and voltages from 0.12 V to 1,200 V, currents up to 12 A, and resistance up to 100 kΩ, with sensitivity at 2,000 Ω/V (6 mA full-scale, adjustable to 3 mA) and at 1,000 Ω/V. It incorporated comprehensive automatic overload protection and a more responsive circuit suitable for frequencies up to audio ranges. The meter movement retained the design for durability, and a key innovation was the introduction of mirror-backed scales to ensure parallax-free readings. Housed in an aluminum case, production of this model continued through with modifications for military use, though pre-war output for all early Avometers remained limited, primarily serving the market.

Post-War Models (1940s–1970s)

The post-war period ushered in a new generation of Avometer multimeters, building on pre-war foundations to meet the demands of expanding industrial and sectors. The Model 7, introduced in 1936, represented a key development with its 20 ranges covering and voltages up to 1000 , currents up to 10 A, resistance from 1 kΩ to 1 MΩ, and from 0.01 to 20 µF (requiring external 50 Hz mains supply of 65–250 ). Its sensitivity of 1,000 Ω/V (doubled via /2 button from 500 Ω/V base) enabled precise high-resistance measurements, making it ideal for industrial electricians handling power systems and circuits. The Model 8 series, launched in 1951, became the defining product of Avometer's golden era, renowned for its robustness and versatility in field environments. The featured approximately 50 measurement combinations across and voltages up to 2500 V, currents up to 10 A, and up to 20 MΩ, with a sensitivity of 20,000 Ω/V and sensitivity of 1,000 Ω/V above 100 V. Subsequent iterations, such as the (introduced around ) and Mark V (1972), incorporated enhancements like fused protection, mirror scales for parallax-free reading, and jack terminals for improved connectivity; the Mark V's redesigned internals supported broader applications, including RF measurements with appropriate probes. Production surpassed 1 million units by 1965, underscoring its widespread adoption. By the 1960s, variants like the military-grade Model 8X met specifications and became standard in service workshops, including those of allied forces. Complementing the Model 8, the Model 9 series emerged in the mid-1960s for technicians, offering refined scales and international symbols for markets. The , circa 1965, provided similar core capabilities to the Model 8—DC voltages from 3 V to 3000 V at 20,000 Ω/V , AC up to 3000 V, currents to 10 A, and resistance to 20 MΩ—but with closer-spaced ranges (3.3:1 ratio) for better low-voltage resolution, suitable for bias measurements in circuits. The Mark IV, introduced in 1969, retained these features while featuring a case and AC up to 15 kHz with less than 2% error, extendable to higher frequencies via accessories for RF work. Avometer also introduced compact variants during this era to enhance portability and specialized use. For applications requiring minimal circuit loading, such as oscilloscope interfacing, the High Resistance (HR) Model 2 (c. 1948–1951) offered 20,000 Ω/V DC sensitivity across 2.5–2500 V ranges and resistance up to 20 MΩ, preventing significant voltage drops in sensitive setups.

Later Models (1980s–2000s)

In the 1980s and 1990s, Avometer continued refining its classic analog designs, with the Model 8 series receiving updates in subsequent marks to enhance and usability for modern electronics. The Model 8 Mk7, produced through the early 2000s, featured a 20,000 Ω/V sensitivity suitable for solid-state circuitry, multi-range measurements for / voltage up to 1000 V, current up to 10 A (including low-current ranges down to 10 μA on some variants), and resistance up to 20 MΩ, maintaining the robust, portable construction of earlier models while incorporating improved overload protection. As digital technology advanced, Avometer introduced its first digital instruments in the late 1970s, such as the DA116 model (1979), a battery-powered multimeter with LCD display for voltage, current, and measurements. Later models, like the AVO830 (introduced 2018), featured auto-ranging, data hold functionality, a 10,000-count (4½-digit) LCD display, and true AC measurements with DC voltage accuracy of ±0.1% + 2 digits. This model supported voltage up to 1000 V, current up to 10 A, to 50 MΩ, , , and testing, with CAT III 1000 V / CAT IV 600 V safety ratings for industrial environments. Late models increasingly incorporated CAT III safety standards and interfaces for basic data logging to computers, enhancing compatibility with professional workflows. Analog production, including the Model 8 Mk7, ended in 2008 as digital alternatives dominated. AVO acquired Megger (Evershed & Vignoles) in 1968, with the group rebranded to Megger in 2002; new Avometer-branded releases diminished, with focus shifting to rebadged Megger digital tools like the AVO300 and AVO400 series, which emphasized ruggedness, auto-ranging, and expanded safety features for field use.

Other Products

Related Test Instruments

In addition to its renowned multimeters, AVO produced a range of specialized electrical test instruments tailored for radio, broadcast, and installation applications, expanding its utility in professional engineering contexts. These devices complemented the core Avometer line by addressing niche measurement needs in high-impedance circuits, insulation integrity, audio signal handling, and wiring safety. The AVO Valve Characteristic Meter (VCM), introduced in the 1950s, served as a high-impedance diagnostic tool for testing vacuum tubes in radio equipment. Models like the Mk II VCM featured input impedances suitable for sensitive tube circuits to minimize loading effects during voltage and emission measurements. This instrument allowed engineers to assess tube performance without altering circuit behavior, essential for troubleshooting broadcast and consumer radio receivers during the post-war era. AVO's insulation testers, often branded under the Megger line following the 1968 acquisition of Evershed & Vignoles, traced roots to hand-crank designs originating in the early but evolved through AVO production from the onward. The AVO Megger models, such as early hand-generated variants, applied test voltages up to 5 kV to evaluate in cables and windings, preventing faults in electrical distribution systems. These portable, generator-powered units became staples for field testing in and settings, with resilient analog scales for readings up to 20 GΩ. For broadcast and audio engineering, AVO developed signal generators and wattmeters from the 1940s to the 1970s, optimized for impedance-matched applications. The AVO All Wave Oscillator (1939–1950s) and later models like the CT378A (1961) generated signals from audio frequencies up to 40 MHz, with outputs calibrated for 600 Ω lines common in broadcast audio chains. Audio wattmeters in this lineup measured power levels in amplifiers and transmission lines, ensuring compliance with standards for radio and equipment by handling balanced 600 Ω terminations without distortion. From the , AVO introduced continuity testers and impedance meters to support electrical installation safety. Low-voltage continuity testers, such as the early Megger variants integrated into AVO lines, used 3–12 V sources to check wiring integrity without risking damage, displaying resistances from 0–30 Ω for fault detection in circuits. impedance meters, like the Series 1 models, measured loop resistances under 1 Ω to verify protective device operation during faults, aligning with emerging wiring regulations for safe in buildings. Following Megger's full integration of AVO in the early , the brand extended to advanced tools like clamp meters. AVO-branded clamp meters, such as the DCM300 series, non-invasively measured currents up to 300 A for load and leakage testing in live installations.

Accessories and Variants

Avometers were typically supplied with a pair of color-coded red and black rubber-covered test leads designed for safe handling of standard voltage measurements up to V. These leads featured flexible to withstand typical field conditions and were compatible with core models such as the Model 8 and Model 40 series. For higher voltage applications, external high-voltage multipliers were available as accessories, extending the measurement range up to 30 kV while maintaining the meter's sensitivity. These multipliers were particularly useful for Model 40 and later variants, allowing safe probing in electrical distribution and industrial settings without compromising accuracy. To facilitate portability, AVO offered carrying cases starting from , often equipped with clips or loops for engineers and technicians. These holsters provided robust protection against dust and minor impacts, with later plastic variants introduced for lighter weight and durability in the post-war period. The cases were tailored to fit principal models like the Model 7 and Model 8, ensuring quick access during on-site . External shunts and multipliers served as key accessories to expand the Avometer's capabilities beyond its built-in limits of 10 A current and 1000 V . current shunts, rated for currents up to 200 A or more, produced a precise 100 mV at , enabling indirect reading on the meter's milliammeter scale via compatible leads. Voltage multipliers, constructed as resistive dividers, allowed extension to higher ranges while preserving the instrument's ohms per volt sensitivity, making them essential for advanced electrical testing. These accessories were compatible with post-war models such as the Model 8 Mk 5 and Model 40, often stored in dedicated compartments within the meter's carrying case. Specialized variants of the Avometer included tropicalized models developed during the era to withstand humid and corrosive environments in tropical climates. These versions featured enhanced sealing, protective coatings, and weatherproof construction to prevent moisture ingress and maintain calibration in demanding field conditions, primarily for and export use with models like the Model 8. Such adaptations ensured reliable performance in regions with high humidity, distinguishing them from standard temperate-zone instruments.

Legacy and Current Status

Cultural and Professional Impact

The Avometer Model 7, introduced in , became a standard tool in electrical training programs during the late , frequently quoted in service manuals and adopted widely in vocational curricula for its reliability in measuring voltage, , and . In professional practice, the Avometer established key benchmarks for analog meter accuracy and sensitivity, particularly with the Model 8's design achieving 20,000 ohms per volt on ranges and 1% accuracy, which influenced competitors in the development of high-performance analog instruments. Avometers have maintained a significant role in , serving as practical tools in vocational schools to demonstrate analog meter movements and their operational principles, in contrast to digital multimeters that prioritize over tactile . For instance, virtual simulations of analog Avometers are employed to train students on scale reading, range selection, and circuit measurement techniques before physical lab work, helping to build foundational skills while minimizing equipment damage. The company's AVO Training Institute, established in the mid-1960s, further extended this legacy by offering specialized electrical training courses .

Production Discontinuation and Collectibility

Production of the iconic Avometer Model 8, the longest-running analog in the series, ceased in 2008 after over a million units had been manufactured since its introduction in 1951. The discontinuation was primarily due to the inability to source certain obsolete components required for its analog , amid a broader shift toward . Following Megger's acquisition of AVO Ltd. in 1987, digital multimeters bearing the AVO name have continued to be produced. The AVO210 model, a hybrid digital multimeter, was available until its discontinuation around 2020. Current models include the AVO850 True-RMS Digital Multimeter, which measures AC/DC voltage up to 1000 V, current to 10 A, to 40 MΩ, and to 4000 μF, maintaining the brand's reputation for durability. Avometer units, particularly Model 8 variants, are commonly traded on platforms like , with prices ranging from £50 for units in fair condition to £300 or more for well-preserved examples complete with original cases and leads. A dedicated restoration community supports the maintenance of older Avometers, with online guides and forums offering step-by-step instructions for tasks like recapping electrolytic capacitors and recalibrating movements in 1950s-era models. These resources, often shared in electronics enthusiast groups such as the UK Vintage Radio Repair and Restoration Discussion Forum, help preserve functionality in instruments over 50 years old. Collectibility drives demand for Avometers, with value influenced by model rarity, condition, and completeness. A mint-condition Mark III Model 8, for instance, can fetch over £200 in private sales or auctions, reflecting its status as a benchmark analog tool. Rare early models from the command premium prices among serious collectors due to their . For contemporary needs, models like the Megger AVO850 provide reliable successors for electricians and technicians.

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