Smectic liquid crystals can be prepared as freestanding films (like a soap bubble on a ring), providing unique opportunities for the study of surface physics. In our lab, we measure the surface tension of such films using a setup depicted in the cartoon above. The gray frame is a brass plate with a rectangular hole. The plate is mounted vertically. A fine suture suspends a small weight of mg and is guided across the rectangular opening by a small groove in the brass frame. A freestanding film is prepared over half of the opening so that the film is bounded on one side by the suture. The surface tension of the film exerts a force on the suture to the left as shown by the arrow. This force is in competition with the downward force of gravity, mg. When the forces are balanced, the suture will map out an arc that is part of a larger circle with radius R. The surface tension, g, the weight, mg, and the radius of curvature, R, are simply related by the equation g = mg/2R where the factor of two arises due to the two free surfaces of the film.