Demonstration of Thomson Effect

Starling’s method of demonstration the Thomson effect. An iron rod ABC is bent into U shape. Its ends A and C are dipped in mercury baths. C1 and C2 are two insulated copper wires of equal resistance wound round the two arms of the bent rod. C1 and C2 are connected in the opposite gaps of a meter bridge. The bridge is balanced. Then the mid-point B of the rod is strongly heated. A heavy current is passed through the rod. Then this current will flowing up the temperature gradient in one arm and down the temperature gradient in the other arms. As a result, one of the coils will be cooled and the other will be warmed. The balance in the bridge will be upset and the galvanometer in its circuit will show a deflection. If the direction of the current is reversed, the deflection in the galvanometer will be reversed.