Maxwell's top, has adjustable center of gravity.

Gyroscopic Procession: Maxwell's Top. Maxwell's Top illustrates conditions for gyroscopic precession. The top is 19 cm. in diameter, and 3.8cm. thick, with a threaded axle that can be adjusted so that the center of gravity of the top is raised or lowered with respect to the pivot point. Spin the top by hand. When the axle is adjusted so that the pivot is above the center of gravity, the top will precess in one direction.When the pivot is below the center of gravity, the top will precess in the opposite direction. If the pivot is set so that it is coincident with the center of gravity, the top will not precess; it will move along any object held against the top of the axle, as well.