h a l f b a k e r y
Not so much a thought experiment as a single neuron misfire.
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Start with a hollow torus (aka a donut or bagel shape, for the mathematically disinclined) with a sealable cap.
Fill it with the liquid and wax used in a regular lava lamp.
Add a light/heat source, at the bottom, but just slightly off center.
The wax gets heated, floats upwards in blobs, cools,
and floats downwards.
Since the heat is supplied off-center, a thermosiphon effect causes the rising wax blobs to move up on one side of the torus, and move down on the other.
Rotating Lava Lamp
This is sort of the inverse idea to what I was describing above (because the lava lamp is in the center hub rather than in the rim), but it is still sort of cool looking, and it does illustrate the mechanical action. [jurist, Feb 10 2010]
||I might be more impressed with this idea if the toroid slowly rotated, like a tire, while the heat source remained stationary within the motorized base. I think you might get more interestingly-shaped blobs with the movement.
||might I most humbly suggest a cold light centred but an extra heat source offset... [+] anyways
||I just thought of this idea and lo! goldbb did too. Exactly as I had conceived it.