Voltmeter - Meaning, Unit, Classification, And FAQs - Vedantu

A voltmeter is an electrical instrument designed to measure the voltage, or electrical potential difference, between two points in an electric circuit. It plays a central role in analyzing both direct current (DC) and alternating current (AC) electric circuits. Voltmeters are available in analog and digital forms, with readings commonly given in volts (V), millivolts (mV), or kilovolts (kV). In modern practice, digital voltmeters are widely used due to their higher accuracy and direct numerical displays, while analog voltmeters involve a pointer moving over a scale.

Working Principle of a Voltmeter

The fundamental principle behind the voltmeter is that it is connected in parallel to the section of the circuit where the voltage is to be measured. This ensures that the instrument experiences the same potential difference as the component under test.

A voltmeter is constructed to have high resistance so that it draws minimal current from the circuit. This design reduces the effect of the measurement on the actual circuit conditions and avoids altering the circuit's current.

Types of Voltmeters

There are several types of voltmeters, each built for specific functions and accuracy levels:

• Analog Voltmeter: Uses mechanical movement (like a pointer and coil) to display voltage.
• Digital Voltmeter: Shows direct numeric values on a digital display.
• Electrostatic Voltmeter: Utilizes electrostatic forces and can directly measure voltage without relying on circuit current.
• Electronic Voltmeter: Employs amplification or rectification and can measure very low or high voltages with minimal circuit loading.
• Multimeter (Volt-Ohm-Milliammeter): Measures voltage, current, and resistance.

Voltmeter Connection and Usage

For accurate results, a voltmeter must always be connected in parallel to the component or section of the circuit under observation. If the voltmeter is incorrectly connected in series, its high resistance can block the current flow, significantly affecting circuit performance and leading to incorrect readings.

Because voltmeters are marked by high resistance, they minimize current draw and circuit disturbance. This feature is crucial for maintaining circuit integrity during measurements.

Key Formulas and Calculations

The most basic relationship used in voltmeter measurements is Ohm’s Law, which links the voltage, current, and resistance in a circuit:

V = IR

Where V is the voltage (in volts), I is the current (in amperes), and R is the resistance (in ohms).

Parameter	Formula	Unit
Potential Difference	V = I × R	Volt (V)
Resistance of Ideal Voltmeter	Approaches Infinity	Ohm (Ω)

Step-by-Step: How to Use a Voltmeter

1. Ensure the circuit is powered off before making any connections.
2. Connect the voltmeter’s positive terminal to the higher potential point and the negative terminal to the lower potential point, across the component to be measured.
3. Switch on the circuit and observe the voltage reading on the scale or digital display.
4. After measurement, power off the circuit and disconnect the voltmeter.

Comparison: Voltmeter, Ammeter, and Multimeter

Instrument	Measured Quantity	Connection in Circuit	Internal Resistance	Common Unit
Voltmeter	Voltage (Potential Difference)	Parallel	High	Volt (V)
Ammeter	Current	Series	Low	Ampere (A)
Multimeter	Voltage, Current, Resistance	Series/Parallel	Variable	V, A, Ω

Solved Example

Suppose a voltmeter is connected across a resistor of 5 Ω, and the current through the resistor is 0.4 A.

Applying Ohm’s Law:

V = I × R = 0.4 × 5 = 2 V

Therefore, the voltmeter reads 2 volts across the resistor.

Practice Questions

1. Why must a voltmeter have high resistance? What happens if it is connected in series?
2. If a voltmeter measures 15 V across a 3 Ω resistor, what is the current through the resistor?
3. List two differences between a voltmeter and a multimeter.

Explore More Resources

• Difference Between Ammeter and Voltmeter
• Voltmeter - Overview and Types

A voltmeter is an essential instrument for everyone studying or working with electricity. Mastering its working, types, and proper usage builds a strong foundation in the analysis and troubleshooting of electrical and electronic circuits.