A voltmeter is an instrument used for measuring

electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...

difference between two points in an electric circuit. It is connected in parallel. It usually has a high

resistance Resistance may refer to: Arts, entertainment, and media Comics * Either of two similarly named but otherwise unrelated comic book series, both published by Wildstorm: ** ''Resistance'' (comics), based on the video game of the same title ** ''T ...

so that it takes negligible current from the circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage measured and can be built from a

galvanometer A galvanometer is an electromechanical measuring instrument for electric current. Early galvanometers were uncalibrated, but improved versions, called ammeters, were calibrated and could measure the flow of current more precisely. A galvano ...

and series

resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias activ ...

. Meters using

amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost th ...

s can measure tiny voltages of microvolts or less. Digital voltmeters give a numerical display of voltage by use of an

analog-to-digital converter In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide ...

. Voltmeters are made in a wide range of styles, some separately powered (e.g. by battery), and others powered by the measured voltage source itself. Instruments permanently mounted in a panel are used to monitor generators or other fixed apparatus. Portable instruments, usually equipped to also measure current and resistance in the form of a

multimeter A multimeter is a measuring instrument that can measure multiple electrical properties. A typical multimeter can measure voltage, resistance, and current, in which case it is also known as a volt-ohm-milliammeter (VOM), as the unit is equipped w ...

, are standard test instruments used in electrical and electronics work. Any measurement that can be converted to a voltage can be displayed on a meter that is suitably calibrated; for example, pressure, temperature, flow or level in a chemical process plant. General-purpose analog voltmeters may have an accuracy of a few percent of full scale and are used with voltages from a fraction of a volt to several thousand volts. Digital meters can be made with high accuracy, typically better than 1%. Specially calibrated test instruments have higher accuracies, with laboratory instruments capable of measuring to accuracies of a few parts per million. Part of the problem of making an accurate voltmeter is that of calibration to check its accuracy. In laboratories, the Weston cell is used as a standard voltage for precision work. Precision voltage references are available based on electronic circuits.

Schematic symbol

In circuit diagrams, a voltmeter is represented by the letter ''V'' in a circle, with two emerging lines representing the two points of measurement.

Analog voltmeter

A moving coil

can be used as a voltmeter by inserting a

in series with the instrument. The galvanometer has a coil of fine wire suspended in a strong magnetic field. When an electric current is applied, the interaction of the magnetic field of the coil and of the stationary magnet creates a torque, tending to make the coil rotate. The torque is proportional to the current through the coil. The coil rotates, compressing a spring that opposes the rotation. The deflection of the coil is thus proportional to the current, which in turn is proportional to the applied voltage, which is indicated by a pointer on a scale. One of the design objectives of the instrument is to disturb the circuit as little as possible and so the instrument should draw a minimum of current to operate. This is achieved by using a sensitive galvanometer in series with a high resistance, and then the entire instrument is connected in parallel with the circuit examined. The sensitivity of such a meter can be expressed as "ohms per volt", the number of ohms resistance in the meter circuit divided by the full scale measured value. For example, a meter with a sensitivity of 1000 ohms per volt would draw 1 milliampere at full scale voltage; if the full scale was 200 volts, the resistance at the instrument's terminals would be ohms and at full scale, the meter would draw 1 milliampere from the circuit under test. For multi-range instruments, the input resistance varies as the instrument is switched to different ranges. Moving-coil instruments with a permanent-magnet field respond only to direct current. Measurement of AC voltage requires a

rectifier A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an inve ...

in the circuit so that the coil deflects in only one direction. Some moving-coil instruments are also made with the zero position in the middle of the scale instead of at one end; these are useful if the voltage reverses its polarity. Voltmeters operating on the electrostatic principle use the mutual repulsion between two charged plates to deflect a pointer attached to a spring. Meters of this type draw negligible current but are sensitive to voltages over about 100 volts and work with either alternating or direct current.

Amplified voltmeter

The sensitivity and input resistance of a voltmeter can be increased if the current required to deflect the meter pointer is supplied by an amplifier and power supply instead of by the circuit under test. The electronic amplifier between input and meter gives two benefits; a rugged moving coil instrument can be used, since its sensitivity need not be high, and the input resistance can be made high, reducing the current drawn from the circuit under test. Amplified voltmeters often have an input resistance of 1, 10, or 20 megohms which is independent of the range selected. A once-popular form of this instrument used a vacuum tube in the amplifier circuit and so was called the vacuum tube voltmeter (VTVM). These were almost always powered by the local AC line current and so were not particularly portable. Today these circuits use a solid-state amplifier using

field-effect transistor The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. FETs ( JFETs or MOSFETs) are devices with three terminals: ''source'', ''gate'', and ''drain''. FETs con ...

s, hence FET-VM, and appear in handheld digital multimeters as well as in bench and laboratory instruments. These largely replaced non-amplified multimeters except in the least expensive price ranges. Most VTVMs and FET-VMs handle DC voltage, AC voltage, and resistance measurements; modern FET-VMs add current measurements and often other functions as well. A specialized form of the VTVM or FET-VM is the AC voltmeter. These instruments are optimized for measuring AC voltage. They have much wider bandwidth and better sensitivity than a typical multifunction device.

Digital voltmeter

A digital voltmeter (DVM) measures an unknown input voltage by converting the voltage to a digital value and then displays the voltage in numeric form. DVMs are usually designed around a special type of

called an integrating converter. DVM measurement accuracy is affected by many factors, including temperature, input impedance, and DVM power supply voltage variations. Less expensive DVMs often have input resistance on the order of 10 MΩ. Precision DVMs can have input resistances of 1 GΩ or higher for the lower voltage ranges (e.g. less than 20 V). To ensure that a DVM's accuracy is within the manufacturer's specified tolerances, it must be periodically calibrated against a

voltage standard A voltage reference is an electronic device that ideally produces a fixed (constant) voltage irrespective of the loading on the device, power supply variations, temperature changes, and the passage of time. Voltage references are used in power supp ...

such as the Weston cell. The first digital voltmeter was invented and produced by

Andrew Kay Andrew F. Kay (January 22, 1919 – August 28, 2014) was a businessman and innovator. He was President and CEO of Kaypro, a personal computer company, which at one time was the world's fourth largest manufacturer of computers, and the largest in ...

of Non-Linear Systems (and later founder of

Kaypro Kaypro Corporation was an American home and personal computer manufacturer based out of San Diego in the 1980s. The company was founded by Non-Linear Systems (NLS) to compete with the popular Osborne 1 portable microcomputer. Kaypro produced a ...

) in 1954. Simple AC voltmeters use a rectifier connected to a DC measurement circuit, which responds to the average value of the waveform. The meter can be calibrated to display the

root mean square In mathematics and its applications, the root mean square of a set of numbers x_i (abbreviated as RMS, or rms and denoted in formulas as either x_\mathrm or \mathrm_x) is defined as the square root of the mean square (the arithmetic mean of th ...

value of the waveform, assuming a fixed relation between the average value of the rectified waveform and the RMS value. If the waveform departs significantly from the sinewave assumed in the calibration, the meter will be inaccurate, though for simple wave shapes the reading can be corrected by multiplying by a constant factor. Early "true RMS" circuits used a thermal converter that responded only to the RMS value of the waveform. Modern instruments calculate the RMS value by electronically calculating the square of the input value, taking the average, and then calculating the square root of the value. This allows accurate RMS measurements for a variety of waveforms. .

References

External links