Centrifugal Eggbeater

Spinning frame that demonstrates equatorial bulge (oblateness).

What it shows:

The rotation of a planet about its axis causes its equator to bulge due to the "centrifugal force" acting on its mass. Here a spinning wire frame simulates the effect.

How it works:

Planets are actually oblate spheroids rather than spheres due to their rotation. This device consists of two spring metal rings mounted on a metal axis. The north pole is free to slide so that, as the frame spins, the hoops flatten and the equator bulges. The axis is hand cranked using a 9-1 gear ratio; a moderate turn of the handle flattens the hoops to half the polar diameter.

figure 1. Centrifugal Eggbeater (crank and C-clamp not shown)

centrifugal eggbeater

Setting it up:

The assembly clamps to the edge of a bench.


This effect causes the equatorial radius of the Earth to be 43km greater than its polar radius. The effect is more pronounced for Jupiter, whose rotation period is under 10 hours. Jupiter's equatorial radius is 71492km, 4638km greater than its polar, giving the planet a distinctly flattened appearance. Our machine seems to be of Russian origin, but Cenco (74435) make essentially the same thing.