A square metal sheet 3.3 cm on a side and of negligible thickness is attached to a balance and inserted into a container of fluid. The contact angle is found to be zero, and the balance to which the metal sheet is attached reads 0.41 N. A thin veneer of oil is then spread over the sheet, and the contact angle becomes 180°. The balance now reads 0.40 N. What is the surface tension of the fluid?