Assertion (A): Manganin and constantan are used for making standard resistors.
Reason (R): Their resistivity has a very weak dependence on temperature.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both (A) and (R) are true and (R) is the correct explanation of (A).
(A) Manganin and constantan are used for making standard resistors. — True. These alloys, like Nichrome, are widely used in wire bound standard resistors.
(R) Their resistivity has a very weak dependence on temperature. — True. The resistivity of a pure metal such as copper rises quite a lot as it warms up. These alloys show only a very small change of resistivity with temperature.
Why R explains A?
A standard resistor must keep the same known resistance even when the room temperature changes or the current warms the wire.
The resistivity of manganin and constantan hardly depends on temperature. So the resistance of a coil made from them changes very little with temperature, and these alloys are chosen for standard resistors.
NCERT (2026-27):
Temperature Dependence of Resistivity – “Some materials like Nichrome (which is an alloy of nickel, iron and chromium) exhibit a very weak dependence of resistivity with temperature …. Manganin and constantan have similar properties. These materials are thus widely used in wire bound standard resistors since their resistance values would change very little with temperatures.”