Understanding Measurement Methods: Null vs. Deflection

Electrical instruments measure quantities using different techniques. Two primary methods are:

• Null Method: This technique involves balancing the unknown quantity against a known standard quantity. The measurement is indicated when the difference between the unknown and standard is zero, often detected by a sensitive galvanometer or other null indicator. This method typically yields high accuracy.
• Deflection Method: In this method, the unknown quantity directly causes the deflection of an indicator (like a pointer on a scale). The magnitude of the deflection corresponds to the value of the quantity being measured. Calibration is crucial for accuracy.

Instrument Analysis for Null Method Usage

Let's examine how each instrument listed operates:

1. Megger (Insulation Tester)

A Megger is primarily used to measure high electrical resistance, particularly insulation resistance. It operates on the deflection principle. It applies a high DC voltage across the resistance being measured and indicates the resistance value based on the resulting current flow. The instrument's scale is calibrated to show resistance directly from this deflection.

2. DC Potentiometer

A DC potentiometer is designed to measure unknown voltages accurately. It works on the null method. The unknown voltage is connected in opposition to a known adjustable voltage. By adjusting the known voltage until there is no current flow through a galvanometer (indicating a null state), the unknown voltage can be determined accurately by comparing it to the calibrated setting of the known voltage source.

3. Kelvin Double Bridge

The Kelvin double bridge is a specialized circuit used for measuring very low resistances accurately. It employs the null method of measurement. By adjusting resistance ratios within the bridge, a balance point (null deflection) is achieved on a sensitive galvanometer, allowing for the precise determination of the low unknown resistance.

4. AC Potentiometer

Similar to its DC counterpart, an AC potentiometer measures unknown AC voltages or currents. It utilizes the null method, balancing the unknown AC quantity against known AC standards using phase-sensitive detection techniques. A null detector (like a vibration galvanometer or electronic amplifier) is used to indicate the balance condition.

Conclusion on Measurement Techniques

Based on the operational principles:

• DC Potentiometer: Uses null method.
• Kelvin Double Bridge: Uses null method.
• AC Potentiometer: Uses null method.
• Megger: Primarily uses the deflection method.

Therefore, the Megger is the instrument among the choices that does not employ a null method of measurement.