If a piece of material whose cross-sectional area is A and thickness is Δ Δ x with a temperature difference Δ Δ T between its faces is observed, heat flows between the two faces in a direction perpendicular to the faces. The time rate of heat flow, Q Δ Δ t , for small Q and small Δ Δ t , is proportional to A × × Δ Δ T Δ Δ x . In the limit of infinitesimal thickness Δ Δ x , with temperature difference Δ Δ T , this becomes H = − − k A ( Δ Δ T Δ Δ x ) , where H = ( Q Δ Δ t ) is the time rate of heat flow through the area A , Δ Δ T Δ Δ x is the temperature gradient across the material, and k , the proportionality constant, is the thermal conductivity of the material. People often use k , λ λ , or the Greek letter κ κ to represent this constant. The minus sign is there because the rate of heat flow is always negative—heat flows from the side at higher temperature to the one at lower temperature, not the other way around.