My compliments to John for his very nice implementation. John was considering measuring small parts also and attaching one of the calibration weights to slow the whole thing down and make it easier to measure. I have been giving this some thought, and since it is probably of (relatively) general interest, thought I'd comment here rather than in private. My general conclusion is that to measure small parts, you need a much weaker spring and smaller calibration bars - on the same order of magnitude as the parts you want to measure (i.e. 1000-2000 g cm^2). Here's why. Let's say you can measure the period accurately to .01 seconds. Sounds small, but when you count 20 periods and divide by 20 it's doable. With my setup, with a 20,000 g cm^2 bar attached, if I change the period by .01, I get a change of approximately 300 g cm^2. (Plug in the period you get for this bar and the period + .01 and see what difference in MOI you get - that's your resolution.) To measure a change of 0.1g in a hammer - the resolution we've become accustomed to - requires measuring accurately to about 1/2000 sec. There are basically two ways to get around this problem. The first is to be able to measure time more accurately. This is doable with electronic sensors and timers (e.g. a photogate). While a knowledgeable person could probably build one cheaply, to purchase one outright is a few hundred dollars. There are very nice ones designed for measuring pendulums in physics labs that could do the job very well. The second way to get around the problem is to use a much weaker spring. If we look at the equation I = T^2 / K - Ib I = moment of inertia of part being measured T = period Ib = MOI of balance K = constant In this equation, K = 4 * pi^2 / kappa, where kappa is the torsional spring constant. We want K to be larger so that larger changes in T result in less change in I, and the way to do this is to make kappa smaller (i.e. use weaker spring). Sounds like interchangeable springs might be a good thing to design in for this purpose. Some S-hooks on the strings where they attach to the springs should make this easy enough. Those wishing to use magnets might want to keep this in mind also. BTW, the implication is that you can also get better resolution for your larger parts by using a weaker spring. -Mark
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