The most common way to change the density of compounds is to change their temperature. Heating a compound activates the molecules so that they spread apart, making the compound less dense. If you have read How Thermometers Work, you know that heating water causes it to expand quite a bit. Cooling the compound down again increases the density.
If you look inside a motion lamp when it's turned off, you'll find a solid waxy compound on the bottom of the globe. This solid compound is only a little denser than the surrounding liquid compound. When you turn on the light at the base of the globe, here is what happens:
- The solid quickly turns into a liquid and expands, giving it a lower density than the surrounding liquid.
- A warm blob is now slightly less dense than the surrounding liquid, so it rises to the top of the globe.
- Because it is farther away from the heat source, the blob cools slightly, becoming more dense than the surrounding liquid (it does not cool down enough to change back into a solid, however).
- The blob sinks to the bottom of the globe, where it heats up enough to rise again.
This is a pretty simple idea, but it's actually fairly complicated to balance all the elements -- the compounds, the heat source and the size of the globe -- so that the blobs are constantly moving around. In fact, the companies that produce commercial motion lamps guard their ingredients very closely, and motion lamp enthusiasts have had a very hard time reproducing the displays you see in commercial models.
In the next section, we'll look at the history of the liquid motion lamp.