The polar liquid molecules in the bulk liquid arrange themselves so that the cohesive forces between the molecules are shared by the neighboring molecules, thus leading to zero net force. This, of course is the most favorable energetic situation. On the surface, however, the molecules lack the screening from the above thus leading to non-zero net force. This force divided by the length along the liquid surface is called surface tension.
Surface tension is directly the force divided by the perimeter of the plate.
A couple of very important points with this method must be notified. First, the plate must be completely wetted before the measurement to ensure that the contact angle between the plate and the liquid is zero. If this is not true the Wilhelmy method is not valid. Secondly, one must be sure that the position of the plate is correct, meaning that the lower end of the plate is exactly on the same level than the surface of the liquid. Otherwise the buoyancy effect must be calculated separately.