Something for the weekend: Measuring the Plank Constant With Lego tells how you can do science experiment with LEGO. A Watt balance looks like a double-armed scale where one weight can be compared to another weight of known mass. Instead of using two arms, a Watt balance only has one arm, brought into balance by a current flowing through a coil. The only major non-Lego parts in the Lego Watt balance described in A LEGO Watt Balance: An apparatus to demonstrate the definition of mass based on the new SI paper are a few coils of wire wound around a PVC pipe and a few neodymium magnets. Check out the paper PDF.