The ends of an element of zinc are kept at a small temperature difference $ \Delta T$ and a small current (I) is passed through the wire. Then, the heat developed per unit time

Question

(a) is proportional to $\Delta\;T$ and $I$ (b) is proportional to $I^3$ and $\Delta T$ (c) is proportional to Thomson coefficient of the metal (d) is proportional to $\Delta T$ only