Education

Voltmeter and Why does voltmeter have High Resistance

The electrical circuits use instruments like ammeter and voltmeter to measure current and voltage respectively. The ammeter is always connected in series and the voltmeter is connected in series.

What is a Voltmeter

The voltmeter is an instrument used to measure the potential difference or the voltage between the two points in the circuit. The resistance of an ideal voltmeter is infinity.

Principle of Voltmeter

The voltmeter works the principle of Ohm’s law. Ohm’s law states that the voltage across the resistor is proportional to the current passing through the resistor. The voltmeter must be always connected parallel to the circuit.

Classification of voltmeter

The voltmeter can be classified based on the following

Based on output

Analog voltmeter and Digital voltmeter

The voltmeter is both analog and digital. In an analog voltmeter, the pointer will move on the scale in proportion to the voltage. In a digital voltmeter, the voltage is displayed as numerical. The digital voltmeter uses an analog to digital converter.

Based on the measurement

DC voltmeter and AC voltmeter

The DC voltmeter has polarities. The positive terminal of the voltmeter must be connected to the higher potential and the negative terminal of the voltmeter should be connected to the lower potential. The AC ammeter does not have any polarity.

Based on construction

Rectifier type voltmeter

The rectifier converts the AC quantity into the DC quantity. Then the DC signal is measured using PMMC voltmeter.

MI voltmeter

MI stands for moving iron. This voltmeter can be used to measure AC voltage and DC voltage

Electrodynamic voltmeter

The electrodynamic voltmeter is used in the measurement of both AC voltage and DC voltage

PMMC voltmeter

The PMMC voltmeter is used to measure DC voltage. The accuracy of the instrument is high.

Induction type voltmeter

Induction type instruments are used in the measurement of AC voltage.

Voltmeter connected in parallel

The voltmeter has a high resistance, therefore connecting the voltmeter in series will reduce the current flow through the circuit. This gives a wrong value of the voltage. The voltmeter is always connected in parallel so that the same voltage drop occurs across the voltmeter.

Multiplier

The range of the voltmeter can be increased by connecting a resistance in series with the voltmeter. The resistor connected in series with the voltmeter is called a multiplier.

Voltage Sensitivity

The voltage sensitivity of the voltmeter is the deflection of the pointer of the voltmeter for the given voltage change. The deflection of the pointer depends on the current, the current in turn depends on the resistance. The voltage sensitivity is given by the unit ohms per volt. The resistance of the voltmeter is equal to the sensitivity times the complete voltage.

Why does a Voltmeter have high resistance?

We know that a voltmeter is always connected in parallel and an ideal voltmeter has infinite resistance. Let us find out why does voltmeter have a high resistance. The voltmeter should measure the voltage without altering the current flow through the circuit. If the voltmeter has low resistance it will draw current and will result in the wrong measurement. Therefore the voltmeter should have high resistance.

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