Two wires of resistances R1 and R2 have temperature coefficient of resistances α1 and α2 respectively . These are joined in series. The effective temperature coefficient of resistance is

At temperature  θR11=R11+α1Δθ  ,  R21=R21+α2ΔθAt  0°C ,  Req=R1+R2At  θ°C ,  Req1=R11+R21=R11+α1θ+R21+α2θ Let equivalent temperature coefficient is  α1At  θ°C ,  Req1=Req1+α1θR1+R21+α1θ=R11+α1θ+R21+α2θα1=R1α1+R2α2R1+R2