Short Answer:

A voltmeter should have high resistance so that it does not draw significant current from the circuit being measured. Since it is always connected in parallel, a high resistance ensures minimal current passes through the voltmeter, preventing any change in the actual voltage of the circuit.

If the voltmeter had low resistance, it would act like a load and allow more current to flow, which could disturb the circuit and give inaccurate readings. Therefore, high resistance in a voltmeter helps maintain circuit stability and ensures accurate voltage measurement.

Detailed Explanation:

Why voltmeter has high resistance

A voltmeter is an instrument used to measure the electrical potential difference (voltage) between two points in a circuit. To correctly measure the voltage without changing the circuit behavior, a voltmeter is always connected in parallel with the component or section of the circuit where the voltage is to be measured.

In a parallel connection, if the voltmeter had low internal resistance, it would allow a large amount of current to flow through it. This would not only change the current distribution in the circuit but could also alter the voltage across the component being measured. This defeats the purpose of voltage measurement.

To avoid such problems, a voltmeter is designed with very high resistance, often in the megaohm range. This ensures that very little current flows through the voltmeter itself, and the circuit under test remains undisturbed.

Main Reasons for High Resistance in Voltmeter:

1. Prevents Circuit Loading:
   A low-resistance voltmeter would draw current and affect the voltage it is supposed to measure. High resistance avoids this by behaving almost like an open circuit.
2. Ensures Accurate Readings:
   High resistance ensures that the voltmeter does not change the voltage across the points it is connected to, providing true voltage values.
3. Reduces Power Drain:
   High resistance means less current draw, which also reduces power loss from the circuit under test.
4. Protects Sensitive Circuits:
   In low-power or high-impedance circuits, even a small additional load could disturb the working of the circuit. High-resistance voltmeters avoid this disturbance.

Practical Explanation:

Consider measuring the voltage across a resistor in a simple circuit. If the voltmeter has low resistance, it will draw current from the circuit, just like another resistor in parallel, and the measured voltage will be lower than the actual voltage. This leads to incorrect measurements and may mislead the analysis of the circuit.

Design Consideration:

Modern digital voltmeters (DVMs) are designed with very high input impedance (often greater than 10 megaohms), ensuring minimal loading. In analog voltmeters, a series resistor (called a multiplier) is used to increase the resistance depending on the voltage range.

Consequences of Low Resistance in Voltmeter:

• Inaccurate voltage readings
• Overloading the circuit
• Disturbance in circuit behavior
• Risk of damage to both circuit and meter

Application:

• Used in electronics labs for testing and fault finding
• Important in measuring voltages in power supplies and batteries
• Used in control systems to monitor circuit performance

Conclusion

A voltmeter must have high internal resistance to measure voltage accurately without affecting the current or voltage of the circuit being tested. High resistance ensures that the voltmeter draws minimal current and maintains the original condition of the circuit, leading to correct and reliable voltage readings. This makes it an essential design feature in both analog and digital voltmeters.