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Voltmeter: Definition and Working Principle
A voltmeter is a measuring instrument which is used to measure voltage across the two terminals in an electrical circuit.
A voltmeter has a very high resistance and it is design in such a way that when connected in parallel to circuit for measuring voltage it does not take appreciable current, so that power consumed is small.
Voltmeter – Working Principle
When a voltmeter is connected in parallel to a circuit element (load), across which the voltage is being measured. Since the voltmeter has a very high resistance, therefore the combination will have almost same impedance that of the load. As we known, in a parallel circuit the voltage being same across each element, so the voltage across the voltmeter and the load is same and hence, the voltmeter measures the voltage across the load.
A simple galvanometer or D’Arsonval meter movement can be converted into a DC voltmeter just by adding a multiplier series resistor RS.
The value of series multiplier can be calculated as follows −
Refer the circuit,
Im = Full scale deflection current, Ifsd
Rm = Internal resistance of meter
V = Voltage of the circuit to be measured
Types of Instruments Used as Voltmeter
The following types of instruments are mainly used as a voltmeter −
Permanent Magnet Moving Coil (PMMC)
It is also called D’Arsonval meter movement.
Used for DC voltage measurement only.
Moving Iron (MI)
It can be used for both AC and DC voltage measurements.
It can also be used for both AC and DC voltage measurements.
Hot Wire and Thermocouple
The calibration of these instruments is also same for AC and DC.
But, these instruments are particularly suitable for AC voltage measurements because their deflection depends upon the heating effect of AC i.e. RMS value of AC, therefore, their readings are independent of the frequency.
Rectifier Type Voltmeter
They can also be used for both AC and DC voltage measurement.
These are used for the measurement of the high voltages.
These instruments used as voltmeters have the advantage that their power consumption is very small.
Note – Nowadays, there are Digital Voltmeters (DVM) are also used, in which the readings are displayed using a seven segment display screen. The picture below gives a look of a typical DVM.
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