I found two different resources which give two different but similar formulae to calculate surface tension for Wilhelmy plate method: in one formula, the surface tension (ST) force is devided by the perimetor of the plate; in the other one, ST force is divided by twice the perimetor. Which one is correct? And can someone please explain me why it is the perimetor? Since the liquid doesn't wet the whole rectangular plate, shouldn't it be twice the length of the plate? (given that the thinness is negligible). Thanx in advance.