Step 1: Temperature coefficient of resistance $\alpha = \dfrac{R_T-R_0}{R_0(T-T_0)}$ — the fractional change in resistance per unit change in temperature.

Step 2: Electrical conductivity $\sigma=1/\rho$ — the reciprocal of resistivity, a measure of how easily a material conducts current.

Step 3: Constantan and manganin are alloys with a very LOW (nearly zero) temperature coefficient of resistance — their resistance remains almost constant even with temperature changes.

Step 4: This makes them ideal for standard resistances, since the resistance value stays reliable/stable regardless of ambient temperature fluctuations during use.