Umair Gaines
Electrical voltage is measured using a voltmeter, which measures the potential difference between two points in an electric circuit. Voltmeter designs vary, with some using a moving coil in a magnetic field, and others using the mutual repulsion between two charged plates to deflect a pointer. Modern voltmeters are usually found within the same physical device as an ammeter and an ohmmeter, known as a multimeter.
| Characteristics | Values |
|---|---|
| Instrument Name | Voltmeter |
| Other Names | N/A |
| Purpose | To measure electric potential difference between two points in an electric circuit |
| Connection | Connected in parallel |
| Resistance | High resistance |
| Types | Analog, Digital |
| Dial | Yes |
| Display | Numerical display of voltage |
| Power Source | Battery or the measured voltage source |
| Accuracy | Depends on the type of voltmeter |
| Use Cases | Monitoring generators or other fixed apparatus, Testing batteries, household wiring, electric motors, and power supplies |
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What You'll Learn
Analogue voltmeters
Electrical voltage is measured using voltmeters. There are two main types: analogue and digital. This response will focus on analogue voltmeters.
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Digital voltmeters
Electrical voltage is measured with a voltmeter. There are two types of voltmeters: analog and digital. This answer will focus on digital voltmeters.
Digital multimeters are another type of digital meter that can measure voltage, in addition to current, resistance, and sometimes other parameters such as capacitance, frequency, and temperature. Multimeters are multipurpose tools, while digital voltmeters are specialized for measuring voltage only.
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Multimeters
Electrical voltage is measured with voltmeters. One type of voltmeter is the multimeter.
There are different types of multimeters, including digital and analog. Digital meters can detect smaller currents than analog meters because they require less current. This means they alter the circuit less. Their resistance as a voltmeter can be greater than an analog meter, and their resistance as an ammeter can be less. Digital meters use an analog-to-digital converter to measure voltage. Analog meters use a galvanometer, which is a coil of wire with a small resistance in a magnetic field, with a pointer attached that points to a scale.
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Ohmmeter
An ohmmeter is a tool used to measure electrical resistance. It can be a standalone device or part of a multimeter, which can also measure voltage and current. Ohmmeters apply a small current to the circuit or component being tested and then measure the resulting voltage. This allows the resistance to be calculated using Ohm's law.
The first ohmmeters were based on a meter movement known as a 'ratiometer', which was similar to the galvanometer type movement. Traditional ohmmeters contained an internal voltage source, such as a battery, that would be connected across the component to be tested, producing a current through the component. The scale of the galvanometer was marked in ohms, as the fixed voltage from the battery meant that as resistance increased, the current through the meter (and hence deflection) would decrease.
More modern ohmmeters have an electronic circuit that passes a constant current through the resistance, and another circuit that measures the voltage across the resistance. These measurements are then digitised with an analog-to-digital converter, and a microcontroller or microprocessor then divides the current and voltage according to Ohm's law. This allows the resistance value to be displayed to the user.
To use an ohmmeter, it should be set to the lowest scale on the resistance setting, which is usually denoted by the Greek letter omega (Ω). When testing a motor, the red ohmmeter cable is connected to the red motor cable, and the black to the black. When testing a fuse, the ends of the ohmmeter cables are connected to the opposite ends of the fuse. A reading between 0 and 2 indicates continuity, meaning the circuit is good. An infinite or OL reading means the circuit is bad.
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Ammeters
An ammeter is an instrument used to measure the electric current flowing in a circuit. Electric currents are measured in amperes, hence the name. Ammeters are connected in series with the circuit in which the current is to be measured.
There are several types of ammeters, including:
- Moving magnet ammeters, which operate on the same principle as moving coil ammeters, but can carry larger currents.
- Moving iron ammeters, which use a piece of iron that moves when acted upon by the electromagnetic force of a fixed coil of wire.
- D'Arsonval galvanometer ammeters, which use magnetic deflection to make the meter needle act in opposite directions for each direction of current.
- Digital ammeters, which have no moving parts and can have accuracies better than 0.1%.
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Frequently asked questions
A voltmeter is an instrument used to measure the electric potential difference between two points in an electric circuit. It is connected in parallel and has a high resistance, so it takes negligible current from the circuit.
There are analog and digital voltmeters. Analog voltmeters use a galvanometer to measure voltage, while digital voltmeters use an analog-to-digital converter. Digital voltmeters are more accurate than analog voltmeters.
A multimeter is an electronic instrument that can measure voltage, current, and resistance. It is a handheld device with a digital display.
