Question

Why are alloys used for making standard resistance coils?

Answer 1

Hint: We should know that a resistance coil is used in an electrical circuit to provide resistance. A resistance coil should not change its resistance by the variation in temperature. So, they must be made from a material having high resistivity and low temperature coefficient. Therefore, they are the factors that should be taken care of while making a resistance coil.

Complete step by step answer:
As we know, a standard resistance coil must not change its temperature with variation in temperature. If this happens then it will not be a standard resistance. Because the value of resistance is a known quantity and if it changes its resistivity according to change in temperature, the known value will differ and it will lead to miscalculations.
So, a standard resistance coil should have a very small temperature coefficient of resistivity. That is those materials will not differ much in resistance while varying the temperature.
Also, we know that resistance of a material increases if its resistivity is high. So, for producing better standard resistance, a resistance coil should be made with materials having high resistivity.
Alloys like constantan and manganin are such materials having a very high value of resistivity, a very small temperature coefficient, and they are usually least affected by atmospheric factors like temperature, moisture, air, etc. so, this is the reason why alloys are used for making standard resistance coils.

Note:
As additional information, we should know that the standard resistance coils are usually made of an alloy called nichrome. It is a non-magnetic alloy that is made up of 80% nickel and 20% chromium. Nichrome has a high resistivity and resistance to oxidation at high temperatures. It is used for making wires and standard resistances.