Hello, I should preface that we are not physicists. We are just a few relatively ordinary guys that committed to building a large pendulum wave (6ft tall, 8ft long). The construction went very well. The frame and the individual pendulum are all strung up. We have a rather ingenious rig that allows us to very easily adjust the length of the wire connecting the pendulum. However, we are running into some issues that hopefully some of you experts will be able to shed some light on. We are using a calculator to determine our 'length' of each pendulum. So for a time period of 60 seconds and an oscillation value of 30 we are provided with a 'length' for the initial pendulum of aprox 100cm. But when we string it up the oscillation only runs for 29. Other considerations: - We are using mason jars as the bobs. So not spherical, roughly cylindrical - We have no idea what 'length' actually refers to. We assumed (wrongly I suspect) that it was the distance between the top of the jar (bob) to the support bar, and that is what we have measured. So, I guess the overall question, after such a lengthy preamble, is where do we measure our 'length' from when using mason jar bobs on a large pendulum wave? Top, middle, or bottom of the jar? And just as an aside, we spent a lot of time eyeballing and counting oscillations by eye, and although this was pretty good in terms of results we believe this exact measuring should be better. Maybe not? Thank you very much!