Short Answer:

A multiplier in a voltmeter is a high-value resistor connected in series with the meter’s internal coil. Its main function is to limit the current that flows through the sensitive coil when high voltage is applied across the voltmeter terminals. This helps in safely extending the range of voltage that the voltmeter can measure.

Without the multiplier, even a small high voltage could damage the internal components of the voltmeter. By using the multiplier resistor, the device can measure large voltages without affecting its accuracy or causing internal damage.

Detailed Explanation:

Function of a multiplier in a voltmeter

A voltmeter is designed to measure voltage, which is the potential difference between two points in a circuit. The core measuring part of an analog voltmeter is typically a moving coil galvanometer that can detect small currents. However, this sensitive device can be damaged if a high voltage is applied directly across its terminals. This is where the multiplier comes into play.

A multiplier is simply a high resistance connected in series with the voltmeter’s coil. Its purpose is to limit the current that flows through the voltmeter by increasing the total resistance of the device. When high voltage is applied to the voltmeter, the multiplier drops most of the voltage, allowing only a small and safe amount of current to pass through the coil. This protects the coil from burning and ensures accurate readings over a wider voltage range.

Working Concept:

Let’s understand this with a simple example. Suppose the coil inside the voltmeter can safely handle only 0.001 amperes (1 mA) of current. Now, if we want to measure 10 volts, then according to Ohm’s law:

V = I × R → R = V / I = 10 / 0.001 = 10,000 ohms

This means we need a total resistance of 10,000 ohms in the voltmeter. If the coil resistance is only 100 ohms, then we need to add a multiplier resistor of 9,900 ohms in series to safely measure 10 volts. The multiplier ensures that only 1 mA flows, which is within the safe limit.

Key Roles of the Multiplier:

1. Protects the Coil:
   The moving coil can carry only a limited current. The multiplier restricts the current and prevents damage to the coil when high voltage is applied.
2. Increases Measuring Range:
   By changing the value of the multiplier resistor, the voltmeter can be calibrated to measure different voltage ranges like 10V, 50V, 100V, or more.
3. Ensures Accuracy:
   A properly calculated multiplier ensures that the voltmeter gives correct readings even when measuring high voltages.
4. Voltage Division:
   The multiplier works as a voltage divider. Most of the applied voltage drops across the multiplier, and only a small voltage appears across the sensitive coil.

Use in Analog Voltmeters:

In analog voltmeters, multipliers are fixed resistors chosen according to the range selector knob. When the user selects a particular voltage range, the voltmeter internally connects the right multiplier in series to measure that voltage safely and accurately.

Use in Digital Voltmeters:

In digital voltmeters, the function of a multiplier is handled by input voltage dividers and protection circuits. Though the components used may be different, the purpose remains the same—to scale down the high input voltage to a manageable level for internal processing.

Conclusion

The multiplier in a voltmeter plays a crucial role in making high-voltage measurements safe and accurate. By offering a large resistance in series, it protects the delicate measuring coil from excess current and allows the voltmeter to work over different voltage ranges. Without this simple but important component, measuring high voltages with precision and safety would not be possible.