Question

It is desired to make a 20 ohm coil of wire which has zero thermal co-efficient of resistivity.To do this, a carbon resistor of resistance 'A' is placed in series with an iron resistor of resistance 'B'. The proportion of carbon and iron is such that A+B=20 ohm for all temperatures near 20 celsius Find A and B if temperature coefficient of resistivity for Iron=5x10^-3 /Kelvin and for Carbon= -0.5x10^-3/Kelvin