The V-I graph for a conductor temperatures T1 and T2 are as shown in the fig.
T2− T1 is proportional to
cos 2θ
sin θ
cot 2θ
tan θ
The slope of V – I graph gives resistance
R1= tan θ , R2= tan 90-θ = cot θ Let say α is temperature coefficient R1=R0 1+αT1 , R2=R0 1+αT2 ⇒R2- R1 = R0 α T2−T1 ⇒cot θ - tan θ = α T2-T1 ⇒ T2-T1 ∝ 2cos2 θ-sin2 θ2cos θsinθ ⇒ T2-T1 ∝ 2cos2θsin2θ ⇒ T2-T1 ∝ cot2θ