Which of the following is better for measuring a very small resistance?

Analysis

The question asks for the most suitable instrument for measuring a very small resistance with high precision. In physics, the choice of instrument depends on the magnitude of the resistance and the required accuracy.

Concept Explanation

• Voltmeter: Used to measure potential difference. While it can be used in a circuit to calculate resistance using Ohm's Law ($R = V/I$), it is not the most precise method for very small resistances due to internal resistance errors.
• Wheatstone Bridge: This is a classic circuit used for measuring medium-range resistances. It works on the principle of a balanced bridge where the ratio of resistors is compared.
• Rheostat: This is a variable resistor used to control current in a circuit. It is not a measuring instrument.
• Potentiometer: This is an instrument used to measure electromotive force (e.m.f) or potential difference by comparing it with a known source. Because it draws no current from the source being measured when balanced (null point), it is extremely accurate. When configured specifically for resistance measurement, it eliminates the errors caused by the internal resistance of the measuring device itself, making it superior for measuring very small resistances.

Step-by-Step Solution

1. Evaluate the options: We eliminate the rheostat immediately because it is a component, not a measuring device.
2. Compare accuracy: The Wheatstone bridge is excellent for medium resistances, but for very small resistances (like those found in short wires or low-value shunts), the contact resistance and lead resistance of the bridge can introduce significant errors.
3. Apply the Potentiometer principle: The potentiometer operates on the "null deflection" method. By balancing the potential drop across the unknown small resistance against a known standard, we avoid the errors associated with current flow and internal resistance. This makes it the most precise tool among the choices for this specific task.

Conclusion

The potentiometer is the most accurate instrument listed for measuring very small resistances because it functions without drawing current from the circuit at the point of balance, thereby minimizing measurement errors.

Exam Tip: Whenever you see a question about "high precision" or "measuring without drawing current," always think of the Potentiometer. It is the "gold standard" for accuracy in these types of measurements.