What it shows:
An accelerated electric charge radiates energy. So according to classical physics, an electron in orbit about an atomic nucleus should emit electromagnetic radiation by virtue of its orbital motion. As it radiates energy, the radius of its orbit decreases. The electron should spiral into the nucleus amidst a burst of radiation in about 10-16 seconds.
How it works:
The tether-ball is a small wooden ball on the end of a 1m cord fixed by a ring clamp to the top of a vertically mounted metal rod. By swinging the ball in a horizontal circle, it will wrap itself around the rod such that its orbital radius decreases and its angular velocity increases. As acceleration increases as the square of the angular velocity, one can also point out that the frequency of the emitted radiation increases as well.
Setting it up:
Clamp a 1.5m clamp stand rod to a bench. A clamp with a ring fixes the end of the cord, but anything that holds the cord firmly and prevents slipping will do. Use a ball of about 5cm diameter, colored loudly so it is visible to all.
A good way to introduce the Bohr atom.