Question

Constantan wire is used for making standard resistance, because it has A. high melting point B. low specific resistance C. high specific resistance D. negligible temperature coefficient of resistance

Answer 1

Hint: Temperature coefficient of resistance is defined as the measure of rise in electric resistance per unit change in temperature.

Formula Used: ${\rho _1} = {\rho _0}\left[ {1 + \alpha \left( {{T_1} - {T_0}} \right)} \right]$
${\rho _1}$: Resistivity at T₁, ${\rho _0}$ : Resistivity at T₀, $\alpha$ : Temperature coefficient of resistivity.

Complete step by step answer:
Constantan is an alloy of copper and nickel. It is commonly known by the names Eureka, Advance and Ferry. The usual composition is 55% of copper and 45% of nickel.
One of the most useful features of the constantan alloy is the low value of thermal variation of resistivity.
The resistivity of the material is not dependent on the physical dimensions of the material. But, it is dependent on the temperature. If we substitute the value of resistance in the above equation, we get:
$R = \rho \dfrac{l}{A}$
Substituting, we get –
${\rho _1} = {\rho _0}\left[ {1 + \alpha \left( {{T_1} - {T_0}} \right)} \right]$
For metals, the value of $\alpha $is positive.
The value of $\alpha $for constantan = $ - 74 \times {10^{ - 6}}{/^ \circ }C$
As you can see, this is fairly, a low negligible number. Thus, the resistivity of constantan changes very little with temperature thus, making the values of resistivity, fairly constant. This property is very helpful in making standard wire-wound resistances for using in electrical circuits.
Hence, the correct option is Option D.

Note:This property of constantan is also, very helpful, for using it in thermocouples. The material constantan has an exceedingly large negative Seeback coefficient above ${0^ \circ }C$
Seeback coefficient is the measure of magnitude of induced thermoelectric voltage due to temperature difference in the material when it is a part of a thermocouple. It is measured in $V/K$(volt/kelvin).