Question

The Kelvin double bridge is used to measure ______ valued resistances and has ______ resistors in its construction.

• A. low ; three-terminal
• B. low ; four-terminal
• C. high ; three-terminal
• D. high ; four-terminal

Hint:

Recall that the Kelvin double bridge fixes the lead and contact resistance error a plain Wheatstone bridge has when measuring very small resistances, and it does this using four-terminal resistors.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question tests two facts about the Kelvin double bridge: what range of resistance values it is meant to measure, and what kind of resistor construction it relies on to do that accurately.

Step 2: Key Formula or Approach:
An ordinary Wheatstone bridge cannot measure very low resistances (below about \(1\ \Omega\)) accurately, because the resistance of the connecting leads and the contact resistance at the terminals become comparable to the resistance being measured, and they add directly to the error. The Kelvin double bridge is a modified Wheatstone bridge built specifically to remove this error for low resistances. It does this by using four-terminal resistors for the standard and unknown resistances, in which two terminals carry the measuring current and a separate pair of terminals pick off the voltage, so that lead and contact resistance drop out of the voltage measurement.

Step 3: Detailed Explanation:
The bridge is called a "double" bridge because it has two sets of ratio arms, an outer pair and an inner pair, connected so that the effect of the resistance of the link joining the low-value standard and unknown resistors cancels out when the two ratios are equal. This construction only works, and only matters, because the standard and unknown resistors are four-terminal devices: the current terminals connect to the bridge supply and the potential terminals connect to the galvanometer and ratio-arm network, separating the current path from the voltage-sensing path.
Options (A) and (C) can be ruled out because "three-terminal" does not describe the current and potential terminal separation the Kelvin bridge depends on; that separation always needs four terminals, two for current and two for potential. Options (C) and (D) can also be ruled out on the resistance range: the Kelvin bridge is used precisely because the plain Wheatstone bridge fails for low resistances, not high ones. High resistances are instead measured with methods such as the loss-of-charge method or a guarded Wheatstone bridge, where lead resistance is negligible compared to the resistance under test.

Step 4: Final Answer:
The Kelvin double bridge measures low valued resistances and uses four-terminal resistors, so the correct option is (B).