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So I am doing a lab to measure the surface tension of tap water, distilled water, and soap solution usin 3 different methods (well 2 for the waters and 3 for the soap).

I did this in a pretty standard way....attached a glass microscope slide to a tensiometer balance...made the slide stick to the surface of a liquid with only a thin film of liquid between them, and saw how far the pointer was depressed on the tensiometer. Then, I removed the liquid, and simply added weights to the slide until it was depressed to the same point - as to produce a force due to gravity equivalent to the force due to surface tension.

From noting how many weights i have attached, I could calculate surface tension usin the relationship Mg (of the weights) = 2 x l x T, where l is the length of the slide, and T the surface tension. the factor of 2 comes form the slide having 2 sides.

My first question - how does the thickness of the glass slide come into effect? It's supposed to have some kind of effect on my measurements....but I ccan't figure out what effect it would have, considering it's not used in the equation to calculate surface Tension based on this bethod...

2nd question - For the soap solution, i measured the surface tension in an additional way thats exactly the same as the above mentioned one, except instead of a glass slide i used a wire frame, which i submerged and it formed a thin film of soap which held it down. After measuring the equivalent force of gravity, it turned out to be about 25% less than the one measured with the glass slide (yet the frame and the glass slide have the same length L). What is the reason that the frame method of measuring surface tension of soap produces a lower value than using the glass slide??? My calculations are different enough for me to think its more than just experimental error...so why are the 2 methods so different?