Two plates identical in size, one of black and rough surface (B₁) and the other smooth and polished (A₂) are interconnected by a thin horizontal pipe with a mercury pellet at the centre. Two more plates A₁ (identical to A₂) and B₂ (identical to B₁) are heated to the same temperature and placed closed to the plates B₁, and A  as shown in figure. The mercury pellet

Smooth and polished plates are poor radiators of heat. Hence, heat coming out from A is small, even though .B being a black and rough plate is a good absorber. Effectively the heat coming to the left of pellet P is small. Black and rough plates are good radiators of heat. Hence, plate B₂ radiates heat to a satisfactory level; however plate A₂, being smooth and polished, is a bad absorber. Effectively, the heat coming to the right of P is also small.