V - I graphs for two materials is shown in the figure. The graphs are drawn at two different temperatures

$R_1=\tan \theta$

$R_2=\tan \left(90-\theta \right)=cot \theta$

$\therefore R_2-R_1=cot \theta -\tan \theta =\frac{\cos \theta }{\sin \theta }-\frac{\sin \theta }{\cos \theta }$

$=\frac{{\cos }^2\theta -{\sin }^2\theta }{\cos \theta .\sin \theta }=\frac{2\cos 2\theta }{\sin 2\theta }$

$=2 cot 2\theta$

$R_2-R_1=2cot 2\theta$

$\therefore \left(T_2-T_1\right)\propto cot 2\theta$