SEARCH Articles Figures Tables A liquid film is held in a wire frame with the right-hand boundary fixed to a freely movable rider. The force F needed to balance the tensiim in the two-sided film is proportional to the length L. Let F 2 L. A displacement of the rider by a distance fix requires work FBx ofiA, where fiA is the increase in area. Thus the tension per unit length in a siiigle suiface, or surface tension cr, is numerically equal to the surface energy per unit area. [Publishers Imprint]