Multimeter glossary

Electrical, Digital multimeters


Accuracy refers to the largest allowable error that occurs under specific operating conditions.

Alternating Current (AC, ac)

An electrical current that periodically changes in magnitude and in direction of the current.


Either half of a cycle of alternating current. It is the time period during which the current increases from zero to its maximum value (in either direction) and decreases to zero.

Alternator (or ac generator)

An electromechanical device which transforms mechanical energy into electrical energy - an alternating current. Very early users called this a dynamo.


An instrument for measuring ac or dc electrical current in a circuit. Unless magnetically coupled, it must be placed in the current path so the flow is through the meter.

Ammeter Shunt

A low-resistance conductor that is used to increase the range of an ammeter. It is shunted (placed in parallel) across the ammeter movement and carries the majority of the current.

Ampere (A)

The unit of measurement for electrical current in coulombs (6.25 x 1018 electrons) per second. One ampere results in a circuit that has one ohm resistance when one volt is applied to the circuit. See Current.


An electrical circuit designed to increase the current, voltage, or power of an applied signal.

Analog Multimeter

A piece of test equipment that can measure volts, ohms, amps, and other electrical characteristics and displays the reading by moving a needle over a fixed scale on a face plate.

Analog-to-Digital Conversion or Converter (ADC or A/D)

The process of converting a sampled analog signal to a digital code that represents the amplitude of the original signal sample.

Audio and audio frequency (AF)

The range of frequencies normally heard by the human ear. Typically, about 20 to 20,000 Hz.

Auto-ranging Multimeter

A digital display meter that after the function is manually selected automatically selects the proper range to display the input signal.

Beta (ß)

The current gain of a transistor when connected in a common emitter circuit, now more commonly called hfe.


A semiconductor device having both majority and minority carriers.


The condition for a reverse biased semiconductor junction when its high resistance, under the reverse bias, suddenly decreases, causing excessive current. Not necessarily destructive.

Bridge Rectifier

A full-wave rectifier in which the rectifier diodes are connected in a bridge circuit to allow current to the load during both the positive and negative alternation of the supply voltage.

Capacitance (C)

Capacitance is the capability to store energy in an electrostatic field. It can be expressed as equal to the charge Q in coulombs that is stored divided by the voltage E in volts that supplied the charge. Capacitance tends to oppose any change in voltage. The unit is farads.

Capacitive Reactance (XC)

The opposition that a capacitor offers to a time changing signal or supplied voltage. Its value is XC = 1/2pfC

Capacitor (C)

A device made up of two metallic plates separated by a dielectric or insulating material. Used to store electrical energy in the electrostatic field between the plates.

Cathode (K)

The negative electrode of a semiconductor diode.

CE Mark

A symbol for European Conformity that indicates that the product was designed and manufactured to mandatory European Safety and EMC/EMI requirements. The Safety requirements cover both electrical and mechanical criteria. EMC is the ability to function properly in the presence electromagnetic signals. EMI makes sure that any electromagnetic emissions are below a level that will interfere with other electronic equipment.

Each of the Installation Categories has different voltage levels. 300, 600, and 1000 Volts are the most common levels of each Category. The higher the rating, the more demanding the testing criteria. Part of the design/testing is how well a product will tolerate transients and other faults external to the product. It also tests how well the case will protect the user in the instance that a fault should happen inside the product. The CE rating of a product means that a product can safely be used to test voltage and current without harming the user in the instance that a fault should happen to the limits described below. This is brief description of each level, but is not an all inclusive description as applications will vary.

Installation Category I

Signal level applications; special equipment, parts of equipment, telecommunications, or other electronic equipment with measures taken to limit transient overvoltages to an appropriate low level.

Installation Category II

Local level, appliances, portable equipment, and other household or products with limited loads. Typically anything that could be plugged into power distribution wall sockets.

Installation Category III

Distribution level or fixed installation applications of permanent connection. This would cover from the service entrance into a building to the power distribution wall sockets.

Installation Category IV

Primary supply levels, outside overhead lines, power cable systems that lead to the service entrance of a building.

Charge (Q)

A measurable quantity of electrical energy representing the electrostatic forces between atomic particles. Electrons have a negative charge.


An inductance which is designed to pass large amounts of dc current. It usually is used in power supply filters to help reduce ripple; although, there are inductances called rf chokes (rfc) which prevent rf from feeding to a circuit.


A complete path that allows electrical current from one terminal of a voltage source to the other terminal.

Circuit Breaker

An electromagnetic switch used as a protective device. It breaks a circuit if the current exceeds a specified value.

Clock Rate

The frequency of oscillation of the master clock, or oscillator, in a system.


The component that is formed when several turns of wire are wound on a cylindrical form or on a metal core.

Counts and digits

Counts and digits are terms used to describe a resolution

Collector (C)

The element in a transistor that collects the moving electrons or holes, and from which the output usually is obtained. Analogous to the plate of a triode vacuum tube.

Color Code

A system in which colors are used to identify the value of electronic components, or other variables, such as component tolerance.

Complementary Metal Oxide Semiconductor (CMOS)

A logic signal that operate from a negative voltage to a positive voltage or at points between the maximum positive and negative voltages. The maximum voltage levels are determined by the manufacturer.


The individual parts that make up a circuit, a function, a subsystem or a total piece of equipment.


A substance through which electrons flow with relative ease.


A special relay for switching heavy currents at power line voltages.


Continuity is the presence of a complete electrical path for current flow.

Controlled Rectifier

A four-layer semiconductor device in which conduction is triggered ON by gate current and OFF by reducing the anode voltage below a critical value.

Coulomb (C)

The unit of electrical charge, made up of a quantity of 6.25 x 1018 electrons.

Crest Factor

The ratio of the Peak value to the RMS value of the waveform.

Current (I)

Current is the rate at which the flow of electrons past a point in a complete electrical circuit. An ampere, or amp, is the international unit used for measuring current. One ampere results when one volt is impressed on a circuit that has a resistance of one ohm.

Data Hold

Holds the displayed reading when button is pressed. The display reading will be released when the HOLD button is pushed again.

Decibel (db)

The standard unit for expressing the ratio between powers P1 and P2. dB = 10 log10 P1 / P2, one tenth of a bel.


The non-conducting material used to separate the plates of a capacitor or for insulating electric contacts.

Digital Multimeter

Multimeters are test equipment that can measure volts, ohms, amps, and other electrical characteristics and displays the reading on an LCD or LED's.

Digital Signal

A signal whose level has only discrete values, like on or off, 1 or 0, +5v or +0.2v.

Digital to Analog Conversion (or Converter) (DAC or D/A)

A circuit that accepts digital input signals and converts them to an analog output signal.


Diodes are a device which has two terminals and has a high resistance to current in one direction and a low resistance to current in the other direction.

Direct Current (DC, dc)

Current in a circuit in one direction only.


The element in field-effect transistor which is roughly analogous to the collector of a bipolar transistor.

Dual impedance digital multimeters

Digital multimeters that allow technicians can safely trouble shoot sensitive electronic or control circuits as well as circuits that may contain ghost voltages and can more reliably determine whether voltage is present on a circuit.

Duty Cycle

Duty cycle is the ratio of time a load or circuit is ON compared to the time the load or circuit is OFF.

Effective Value

The value of ac current that will produce the same heating effect in a load resistor as the corresponding value of dc current.


A form of energy produced by the flow of electrons through materials and devices under the influence of an electromotive force produced electrostatically, mechanically, chemically or thermally.

Electrolytic Capacitor

A capacitor whose electrodes are immersed in a wet electrolyte or dry paste.

Electromotive Force (E)

The force which causes an electrical current in a circuit when there is a difference in potential. Synonym for voltage.


The basic atomic particle having a negative charge that rotates around a positively charged nucleus of an atom.

Electrostatic Field

The electrical field or force surrounding objects that have an electrical charge.

Emitter (E)

The semiconductor material in a transistor that emits carriers into the base region when the emitter-base junction is forward biased.


Any deviation of a computed, measured, or observed value from the correct value.

Farad (F)

The basic unit for capacitance. A capacitor has a value of one farad when it has stored one coulomb of charge with one volt across it. See capacitance.

Field Coil

An electromagnet formed from a coil of insulated wire wound around a soft iron core. Commonly used in motors and generators.

Field-Effect Transistor (FET)

A 3-terminal semiconductor device where current is from source to drain due to a conducting channel formed by a voltage field between the gate and the source.


The heated element in an incandescent lamp of vacuum tube.


A circuit element or group of components which passes signals of certain frequencies while blocking signals of other frequencies.


The ability to emit light when struck by electrons or other radiation.

Forward Resistance

The resistance of a forward-biased junction when there is current through the semiconductor p-n junction.

Forward Voltage (or bias)

A voltage applied across a semiconductor junction in order to permit forward current through the junction and the device.

Frequency (F or f)

Frequency is the number of complete cycles per second in a periodic waveform.

Gain (G)

  1. Any increase in the current, voltage, or power level of a signal.
  2. The ratio of ouput to input signal level of an amplifier.

Ghost Voltages

Ghost voltages are voltages that are present when having energized circuits and non-energized wiring located in close proximity to each other

Ground (or Grounded)

The common return path for electric current in electronic equipment is called electrical ground. Also, referred to as a reference point connected to or assumed to be at zero potential with respect to the earth ground.

Henry (H)

The unit of inductance. The inductance of a coil of wire in henries is a function of the coils size, the number of turns of wire and the type core material.

Hertz (Hz)

Once cycle per second.

Impedance (Z)

In a circuit, the opposition that circuit elements present alternating current. The impedance includes both resistance and reactance.

Inductance (L)

The capability of a coil to store energy in a magnetic field surrounding it which results in a property that tends to oppose any change in the existing current in the coil.

Inductive Reactance (XL)

The opposition that an inductance offers when there is an ac or pulsating dc in a circuit. XL = 2pfL

Input Impedance

Input impedance is the impedance seen by a source when a device or circuit is connected across the source.


The region separating two layers in a semiconductor material, e.g. a p-n junction.

Junction Transistor

A PNP or NPN transistor formed from three alternate regions of p and n type material. The alternate materials are formed by diffusion or ion implantation.

Leakage (or Leakage Current)

Leakage current is current that flows around or through a device or circuit.


Any component, circuit, subsystem or system that consumes power delivered to it by a source of power.

Logic High/Low

Checks the logic level of TTL or CMOS LOGIC signals. Connect black test lead / COM input to the common bus of the logic circuitry. Connect red test lead / V-W input to the point to be tested. A logic '1' level (high pulse), is indicated by the (up arrow) symbol in the display and a logic' 0' level (low pulse) by the (down arrow) symbol and a 40ms beep tone.


A closed path around which there is a current or signal.

Manual ranging Multimeter

A digital display meter that after the function is manually selected the proper range must be selected to display the input signal.

Megohm (MΩ)

A million ohms. Sometimes abbreviated meg.

Microampere (mA)

One millionth of an ampere.

Microfarad (mfd, MFD, or mfd)

One millionth of a farad.

Milliampere (mA)

One thousandth of an ampere.

Millihenry (mH)

One thousandth of an henry.

Milliwatt (mW)

One thousandth of a watt.


Records the minimum and maximum values of the input signals, while displaying the current value. The meter beeps each time a new MIN or MAX value is recorded. See digital multimeter buttons.

NPN Transistor

A bipolar transistor with a p-type base sandwiched between an n-type emitter, and an n-type collector.

N-type Semiconductor Material (N)

A semiconductor material in which the majority carriers are electrons, and there is an excess of electrons over holes.

Ohm (Ω)

The unit of electrical resistance. A circuit component has a resistance of one ohm when one volt applied to the component produces a current of one ampere.


The sensitivity rating for an analog voltmeter. Also expresses the impedance (resistance) presented to a circuit by the meter when a voltage measurement is made.

Ohm's Law

Ohm’s Law is a formula used to calculate the correlation between voltage, current and resistance.

Open Circuit

An incomplete path for current.


A sustained condition of continuous operation where the circuit outputs a constant signal at a frequency determined by circuit constants and as a result of positive or regenerative feedback.


The maximum positive or negative value of a sine wave.

Peak Hold

Holds the "Peak" value of the signal present in the display register. The display can be updated (higher only) at "Peak" value as long as leads are connected but will hold the "Peak" reading when leads are removed.

Peak Min/Max

Peak Min / Max captures intermittent or transient events that occur on a monitored signal. Captures the highest value in a very short duration (microseconds).

Peak to Peak

The value of a signal from the maximum positive point to the maximum negative point.

Pi (p)

The mathematical constant which is equal to the ratio of the circumference of a circle to its diameter. Approximately


Picofarad (pf)

A unit of capacitance that is 1 x 10-12 farads or one millionth of a millionth of a farad.


A crystal property which causes a voltage to be developed across the crystal when mechanical stress is applied, or vice-versa.

PNP Transistor

A bipolar transistor with an n-type base sandwiched between a p-type emitter and a p-type collector.


The description of whether a voltage is positive or negative with respect to some reference point.

Potential Difference

The voltage difference between two points, calculated algebraically.

Power (P)

The time rate of doing work.

Power (Reactive)

The product of the voltage and current in a reactive circuit measured in volt-amperes (apparent power).

Power (Real)

The power dissipated in the purely resistive components of a circuit measured in watts.

Power Supply

A defined unit that is the source of electrical power for a device, circuit, subsystem or system.


Precision is a multimeter’s ability to provide the same measurement repeatedly

Probe Hold

Holds & updates the displayed reading (both higher or lower) as long as test leads are connected, but will hold the display reading when test leads are removed. Select Probe Hold before taking a measurement. The meter beeps to indicate that a stable measurement has been recorded.

P-type Semiconductor Material (P)

A semiconductor material in which holes are the majority carriers and there is a deficiency of electrons.

Range Lock

Locks the display in the currently displayed range . Each subsequent push of the button moves to a higher range. From highest range the meter returns to the lowest range. If the measurement is greater than the selected range can display, an 'Overload' indication will be displayed.

Reactance (X)

The opposition that a pure inductance or a pure capacitance provides to current in an ac circuit.


The process of converting alternating current into pulsating direct current.

Relative Mode

The measurement is stored as a reference value and the display is reset to zero. The reference value is now deducted from subsequent measurements and only the difference is displayed. Take a measurement first and then activate Relative Mode, while the measurement is displayed. See digital multimeter buttons.


A device in which a set of contacts is opened or closed by a mechanical force supplied by turning on current in an electromagnet. The contacts are isolated from the electromagnet.

Resistance (R)

Resistance is a measure of the opposition to current flow of electrons in an electrical circuit. It results in loss of energy in a circuit dissipated as heat.

as heat.


A circuit component that provides resistance to current in the circuit.


Resolution is the smallest increment a multimeter can detect and display

Reverse Current

The current when a semiconductor junction is reverse biased.

Root-Mean-Square (RMS)

See effective value. The RMS value of an ac sinusoidal waveform is 0.707 of the peak amplitude of the sine wave.


One of the materials falling between metals as good conductors and insulators as poor conductors in the periodic chart of the elements.


A parallel circuit branch, see Ammeter shunt.


In electronics, the information contained in electrical quantities of voltage or current that forms the input, timing, or output of a device, circuit, or system.

Silicon Controlled Rectifier (SCR)

A semiconductor diode in which current through a third element, called the gate, controls turn-on, and the anode-to-cathode voltage controls turn-off.

Sine (sinusoidal) Wave

A waveform whose amplitude at any time through a rotation of an angle from 0° to 360° is a function of the sine of an angle.

Step-down transformer

A transformer in which the secondary winding has fewer turns than the primary.

Step-up Transformer

A transformer in which the secondary winding has more turns than the primary.

Thermal multimeter

Thermal multimeter is a professional digital multimeter with an integrated infrared camera (also called a thermal imager). A thermal multimeter is ideal for electrical applications where you need to quickly locate a problem and then test standard electrical parameters.


A set of coils wound on an iron core in which a magnetic field couples energy between two or more coils or windings.


A three-terminal semiconductor device used in circuits to amplify electrical signals or to perform as a switch to provide digital functions.

Transistor Transistor Logic (TTL)

A logic signal that operates between 0 and +5 volt limits.

True RMS

A true-RMS device is a tool that can measure alternating current or AC voltage. The True Root Mean Square value of a waveform = square root of the AC RMS component squared plus DC component squared. This method of measurement should be used for signals that are not sinusoidal and centered at zero. Most TRMS meters have a Crest Factor limit of about 5:1 for accurate measurements.

Turns Ratio

The ratio of secondary winding turns to primary winding turns of a transformer.


A line representing the magnitude and time phase of some quantity, plotted on rectangular or polar coordinates.

Voltmeter vs Multimeter

A voltmeter measures the difference between point a and point b in an electrical circuit. A multimeter combines the testing capabilities of a few single-task meters like the voltmeter (for measuring volts), ammeter (for measuring amps) and ohmmeter (for measuring ohms). Multimeters can take electrical readings for voltage, current, and resistance, in addition to other specialized features.

Voltage or Volt

Voltage is a unit of electromotive force that causes current when included in a closed circuit. One volt causes a current of one ampere through a resistance of one ohm.

Voltage Drop

The difference in potential between two points caused by a current through an impedance or resistance.

Watt (W)

The unit of electrical power in joules per second, equal to the voltage drop (in volts) times the current (in amperes) in a resistive circuit.