Key Difference: An ammeter is an instrument that is used to measure the currents in a circuit. A voltmeter is an instrument that is used to measure the voltage between two points in a circuit.

There are two different ways in order to measure electricity; currents and voltages. Devices such as ammeters and voltmeters, which are based on the galvanometer, a device used to detect small currents, are used to measure electricity. While ammeters are used to measure the currents, the voltmeter is used to measure the voltage. Both the devices are different in terms of functionality and circuit placement.

An ammeter is an instrument that is used to measure the currents in a circuit. The currents are measured in amperes (A). Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. The early ammeters needed to be aligned with the Earth’s magnetic field in order to work properly, though new ammeters can be mounted on any circuit in order to provide current measurements. In order to get a reading with an ammeter, the circuit must be unplugged in order to attach the ammeter to the circuit. For applications where disconnecting the circuit is problem, an alternative type of ammeter is available, known as the contactless ammeter.

In order for an ammeter to measure a current, the current must pass through the ammeter and hence it must be placed in a series mode inside the circuit. The polarities must correspond, the positive and negative polarity must match up with the positive and negative on the circuit. Though ideally, ammeters should have zero resistance, in actuality it has relatively low resistance compared to voltmeters. If the resistance is too high, it may block too much current and affect the currents in the circuit and alter the readings. If an ammeter is accidently wired in parallel with a voltage source, it could cause a short circuit and result in blowing a fuse. An ammeter reading may not always be accurate as a lot of factors, such as failing of a diode in the alternator rectifier, or slipping of the alternator belt, could alter the reading.

A voltmeter is an instrument that is used to measure the voltage between two points in a circuit. The early voltmeters were based on moving coil galvanometers and were created by inserting a resistor in series with the instrument. It uses a small coil of fine wire suspended in a strong magnetic field. When an electric current is applied, the galvanometer's indicator rotates and compresses a small spring. A series resistance is also added so that the angular rotation becomes proportional to the applied voltage. Digital voltmeters were invented by Andrew Kay of Non-Linear Systems in 1954. Unlike ammeters, the circuit does not need to be unplugged in order to attach a voltmeter.

In order to for a voltmeter to measure the voltages, it does not require current to be passed through it. It is placed parallel to the circuits they are expected to measure. Polarity is also expected to be observed in the placement of voltmeters. Ideally voltmeters should have infinite impedance, but this is not the case with actual voltmeter; they have a finite resistance value. While voltmeters should not draw any current from the circuit, they require currents in order produce the repulsive magnetic field. The currents drawn from the circuit can be minimized by employing amplifiers for a more accurate read. If the internal resistance of a voltmeter is too small it will not block enough current and provide a faulty reading. Voltmeter’s accuracy is affected by many factors, including temperature and supply voltage variations. Voltmeters are easily and much safer to install and also provide a more accurate reading compared to ammeters.

Multimeters can also be utilized in place of both ammeters and voltmeters. A multimeter is an instrument that can be used to measure both, currents as well as voltages. It can also perform as ohmmeters, a device used to measure resistance. Multimeters are available in analog or digital format.

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