Multimeter

How it works

The user sets the selector to the quantity and range to be measured, connects the test leads and reads the result. Voltage is measured by placing the leads across the circuit. Current is measured by placing the meter in series with the circuit, through a low-resistance shunt in the meter. Resistance is measured by passing a small current from the meter’s own battery through the part and reading the result on the resistance scale or display.

Analog meters move a pointer with a moving-coil movement. Richard’s Radios, a collector’s history of the AVO line, says the moving-coil type suited the multimeter because of its sensitivity, damping and long scale arcs, and notes that AVO resistance ranges read the current through the unknown resistance on the assumption that the battery voltage stays constant. Digital meters convert the input to a number. Fluke says its 1977 model 8020A used a custom complementary metal oxide semiconductor analog-to-digital converter chip to drive a liquid crystal display.

Fluke’s application notes describe two input behaviors that matter in use. Voltage inputs have a high input impedance, greater than 1 megohm on most digital meters, so that they have little effect on the circuit. Current inputs are a shunt of about 0.01 ohm, so a meter set to measure current and connected across a voltage source is a near short circuit and depends on its fuse.

Anatomy

Display or scale: A pointer over a printed scale on analog meters; a numeric display on digital meters.
Selector switch: Selects the quantity and range. On the AVO meters the collector’s history lists two range switches and two terminals as features that survived from the first model to the last.
Input jacks: A common jack and separate jacks for voltage and resistance and for current; the current jacks are fused.
Test leads and probes: Insulated flexible leads ending in probes; Fluke notes that lead construction and rating matter for both accuracy and safety.
Fuses: High-energy fuses with a stated interrupting rating protect the current inputs.
Battery: Powers the resistance ranges on analog meters and the display on digital meters.

Variants

Analog multimeter (volt-ohm-milliammeter): Pointer on a scale. The Science Museum Group holds AVO models from about 1926 to 1981, including the model 7 of 1956 and the model 8 Mark 5 of 1981.
Digital multimeter: Numeric display. Fluke introduced the bench model 8000A in 1972 and the hand-held 8020A in 1977; AVO’s digital models in the Science Museum Group collection include a Nixie-tube DA112 of the 1960s and an auto-ranging liquid crystal DA117 of the 1980s.
True-rms meter: Measures the effective value of non-sinusoidal alternating signals. Fluke describes most digital meters today as measuring the ac-coupled true-rms signal and rejecting any dc component.
Dual-impedance meter: Adds a low-impedance input position (Fluke calls it Auto-V/LoZ) that reduces false readings from ghost voltage.
Clamp multimeter: A multimeter with a hinged current-sensing jaw; O*NET lists clamp-on multimeters among the tools used. See the clamp meter entry.

Specifications

Input impedance, voltage ranges · megohms: greater than 1Fluke’s description of most digital multimeters sold for industrial, electrical and electronic testing.
Measurement category rating · CAT and volts: CAT III 600 V or 1000 V and/or CAT IV 600 VFluke recommends choosing a tool rated to the highest category and voltage at which it could possibly be used, most often 600 or 1000 volt CAT III and/or 600 volt CAT IV.
Fuse interrupting rating · amperes: 10,000 to 17,000The ratings Fluke states for the fuses in its testers.
Test lead resistance, good leads · ohms: 0.5 or lessFluke’s check of leads by touching the probe tips together in the ohms function.

Made to

IEC 61010-1
IEC 61010-2-033
IEC 61010-2-030

Safety and care

Safety

Match the meter and leads to the measurement category and voltage of the circuit. Fluke’s application note states that a CAT III 600 V meter and a CAT II 1000 V meter are both tested with a 6000 V transient but with different source impedance (2 ohms against 12 ohms), so the voltage figure alone does not show the protection.
Fluke notes that a manufacturer can self-certify that a meter meets a category and recommends looking for the listing symbol and number of an independent testing laboratory.
Never connect the leads across a voltage source with the leads in the current jacks. Fluke calculates that a CAT III 1000 V meter with about 0.1 ohm in the current path would pass 10,000 amperes on a 1000 volt source and says the fuse must have the interrupting rating and voltage rating the manufacturer specifies.
OSHA’s rule 29 CFR 1910.333(b)(2)(iv)(B) requires a qualified person to use test equipment to verify that circuit parts are deenergized, including a test for induced voltage and backfeed, and for circuits over 600 volts nominal requires the test equipment to be checked before and after the test.
Fluke’s working practice for live measurements is to attach the ground clip first, then touch the hot lead, and to remove the hot lead first and the ground lead last.

Care

Inspect leads before each job for nicked or cracked insulation and for gaps where the insulation meets the plug or probe; Fluke notes that PVC and silicone insulation can dry out and crack with age.
Replace a blown fuse only with the type the manufacturer specifies for amperage, interrupting rating and voltage rating.
Discharge any capacitor in a circuit before taking a resistance reading.

Tips

Check the leads by reading their resistance
Set the meter to ohms and touch the probe tips together; Fluke says good leads read about 0.5 ohm or less. Wiggle the leads while watching the display to find an intermittent break, and note that the continuity beeper of some meters sounds at up to 15 ohms and so can pass a poor lead.1
Use the low-impedance position to tell ghost voltage from real voltage
Ghost voltage appears when an energized conductor lies beside unused wiring, for example in the same conduit, and the pair acts as a capacitor. Fluke says the low-impedance (Auto-V/LoZ) input loads the circuit so that a ghost reading drops away, and recommends it when readings are suspect or when testing for the presence of voltage.1
Prove the meter before and after a no-voltage reading
Fluke’s three-point method is to test a known live circuit, test the target circuit, then test the known live circuit again. Where no reliable live circuit is at hand, Fluke suggests a portable proving unit for the first and last steps.1
Use min/max to catch a fault that comes and goes
Meters with a peak min/max function capture events as short as 250 microseconds and hold the highest and lowest values while the leads stay connected to the circuit. Fluke advises pressing HOLD before removing the leads, because otherwise the minimum value is lost and the average is compromised.1
Compare peak to rms to judge the waveform
For a sine wave the peak is 1.414 times the rms value, so a 120 volt rms signal peaks at about 169.7 volts. Fluke says a ratio below 1.414 indicates a flat-topped signal and a ratio above it a more peaked one.1
Choose ac or dc function to separate the two components
Fluke explains that most meters measure the ac-coupled signal and reject dc. It recommends the ac function on a power system where only ac should be present, and the dc function to find a failed diode in a motor-drive rectifier, which shows as a dc reading.1
Expect an over-range display before the probes touch anything
In the resistance function a digital meter begins measuring at once, so with the leads apart the display reads OL. Fluke also instructs the user to discharge any capacitor in the circuit before a resistance reading.1
Lead metal affects very small readings
Fluke notes that leads made with dissimilar metals can act as a thermocouple and add a voltage of their own, an error that is hard to trace when measuring millivolts or very low resistance.1

History

Earliest documented

1920s (first AVOmeter; sources give 1920 and 1923, see disputed)

London, England

The Science Museum Group describes AVOmeters as the first multi-purpose electrical measuring device, developed in the 1920s by Donald Macadie, head of General Post Office Telephones in London, combining amperes, volts and ohms in one instrument. The Group holds Macadie’s papers on the AVOmeter, dated 1922–1953, and an AVO model 1 cataloged as made about 1926.1 2 3 4

The multimeter exists because three separate instruments were too much to carry. The Science Museum Group says Donald Macadie, head of General Post Office Telephones in London, combined amperes, volts and ohms in one device, and Fluke’s history says he did so because he was tired of carrying a set of tools for telecom work. The collector’s history says the alternative in the 1920s was three large instruments with shunt resistors and batteries. The British name AVO is the three quantities, according to both Fluke and the collector’s history.

The instrument first extended its range, not its form. The original AVOmeter read direct current and resistance; the copper-oxide instrument rectifier of the early 1930s added alternating current and voltage and, in Fluke’s account, raised the number of ranges from 13 to 20. The Science Museum Group catalogs the Universal Avometer as made from 1935 to 1978 and holds a model 8 Mark 5 made in 1981. The collector’s history says the mirror scale, two range switches and two terminals of the first design lasted through the last model 8, whose production it dates to 2008.

The digital meter changed the display and the market. Fluke says engineers tried to build a solid-state digital voltmeter in the 1950s but that semiconductors became cheap enough for practical designs only in the 1970s. AVO had a Nixie-tube digital model in the 1960s. Fluke’s 8000A of 1972 and 8020A of 1977 brought the digital meter to the bench and then to the pocket, and Fluke credits the 8020A’s custom CMOS converter chip and nine-volt battery life of 200 hours for its success, with more than 250,000 sold by 1984. These are Fluke’s own figures.

Safety became part of the design. Fluke says it worked with GE laboratories from 1972 on transient protection above 8000 volts. IEC 61010-1 sorts circuits into overvoltage categories by distance from the power source, and Fluke’s application note gives the test transients, for example 6000 volts through 2 ohms for CAT III 600 V and 8000 volts for CAT IV 600 V. IEC’s third edition of 2010 extended the standard to non-professional users, and a separate part, IEC 61010-2-033, covers hand-held multimeters themselves. The fuse is part of the same concern: Fluke warns that a wrong fuse can fail violently when the leads are across a source.

The digital meter did not remove the older tools but changed what they were used for. Fluke’s notes say older solenoid testers and analog meters have low input impedance and are not fooled by ghost voltage, and that dual-impedance digital meters were added to give the same behavior on demand. O*NET’s Tools Used table lists multimeters for 6 of the 9 occupations mapped to this tool in the archive’s crosswalk.

Timeline

1920
Fluke’s history page credits Donald Macadie, a British Post Office engineer, with inventing the first multimeter in this year, saying he wanted one tool in place of several for work on telecom lines.1
1922–1953
Date range of the papers of Donald Macadie in the Science Museum Group collection, which include material on the AVOmeter, the automatic coil winder and a Post Office postal order sorting machine.1 2
1923
According to a collector’s history, the Automatic Coil Winder and Electrical Equipment Company was founded on 23 May 1923 and the first AVOmeter went on sale in 1923 as a dc-only instrument.1
c. 1926
An AVO model 1, serial number 14425, is cataloged by the Science Museum Group as manufactured about 1926.1
early 1930s
Copper-oxide instrument rectifiers allow the AVOmeter to measure alternating current and voltage; Fluke says the ranges rose from 13 to 20 and credits Westinghouse with the first universal meter, and the collector’s history dates the rectifier to the early 1930s.1 2
1935–1978
Production span cataloged for the Universal Avometer made by the Automatic Coil Winder and Electrical Equipment Co. Ltd.1
1948
John Fluke Engineering Company is founded; Fluke’s own accounts place its first product as a highly accurate benchtop power meter sold first to General Electric.1 2
1956
AVO Limited makes the Avometer model 7, cataloged by the Science Museum Group with the date 1956.1
1960–1969
AVO Limited builds the digital Avometer DA112 with a Nixie tube display.1
1972
Fluke introduces the 8000A, which it calls the first low-cost portable bench digital multimeter, and works with GE high energy laboratories on transient protection of more than 8000 volts.1
1977
Fluke introduces the 8020A hand-held digital multimeter at US $169; it was retired in 1984 after more than 250,000 sales, by Fluke’s account.1 2
1980–1989
AVO Limited makes the DA117 auto-ranging digital multimeter with a liquid crystal display, and in 1981 the analog model 8 Mark 5 still appears in the Science Museum Group record.1 2
2010
IEC publishes the third edition of IEC 61010-1, widening its scope to all locations where such equipment may be used, professional and non-professional.1
2012
IEC publishes IEC 61010-2-033 on 4 April 2012, specifying safety requirements for hand-held meters whose primary purpose is measuring voltage on a live mains circuit; the IEC Webstore record says it is to be read with IEC 61010-1:2010.1

Where the sources disagree

When and by whom was the first multimeter made?

1920, by Donald Macadie, described as a British Post Office engineer, who is credited with inventing the first multimeter.1
Fluke
Developed in the 1920s by Donald Macadie, head of General Post Office Telephones in London; the AVOmeter was the first multi-purpose electrical measuring device.1
Science Museum Group
The first AVOmeter went on sale in 1923, made by the company founded on 23 May 1923.1
Richard’s Radios (collector’s history)

Where was Fluke’s founding workshop?

A basement workshop in Springfield, Connecticut.1
Fluke press release, 2023
A basement workshop in Springdale, Connecticut.1
Fluke 70th anniversary timeline

Gaps in the record (6)

The measurement-category content of IEC 61010-2-033 (the former abstract at iTeh, a standards reseller, said categories III and IV were defined in an annex) was removed because the IEC Webstore record does not state it and the standard text was not read; the category figures above rest on Fluke and IEC 61010-1 sources only.
Macadie’s patent text has not been retrieved. A secondary index lists it as British patent GB 200977 of 1923, but the patent has not been read and is not cited.
No contemporary AVO manual or catalog of the 1920s has been read, so the first AVOmeter’s ranges, accuracy and price are not sourced.
The Fluke accounts of its own firsts (8000A, 8020A) are company statements; no independent source was located.
The year in which CAT III and CAT IV ratings first appeared on multimeters is not sourced; Fluke’s history page does not date that entry.
The first digital multimeter by any maker has not been sourced independently of Fluke’s claims.