Lava lamp

A lava lamp is a novelty item typically used for decoration rather than illumination. The gentle flow of randomly-shaped blobs of wax suggests the flowing of lava.

How it works
The lamp consists of an illuminating bulb, and a glass bottle containing a transparent liquid, translucent wax, and a metallic wire coil. The glass bottle sits on top of the bulb, which heats its contents.

The wax is slightly denser than the liquid at room temperature, and slightly less dense than the liquid under marginally warmer conditions. This happens because the wax expands more than the liquid when heated.

The light bulb heats the container at the bottom, and due to heat exchange with the atmosphere, the container eventually dissipates the introduced heat. This method of heat transfer is called convection.

Wax at the bottom heats until it melts, and eventually becomes less dense than the liquid above it. At this time, a portion of the wax rises towards the top of the container. Near the top, away from the heat source, the wax cools, contracts, and as its density increases it begins to fall through the liquid towards the bottom of the container again. The difference in temperature between the top and bottom of the globe is only a few degrees.

One mass of wax may rise as another falls. The metal coil at the bottom helps to overcome the surface tension of the individual wax droplets, causing the descending blobs to agglomerate into a single molten wax mass at the bottom of the container. The cycle of rising and falling wax droplets continues so long as the bottom of the container remains warm and the top of the container remains cool. Operating temperatures of lava lamps vary, but are normally around 60 °C (140 °F). If too low or too high a wattage bulb is used in the base, the "lava" ceases to circulate, either remaining quiescent at the bottom (too cold) or all rising to the top (too hot).

Chaotic behaviour makes the movement of the wax unpredictable&mdash;hence lava lamps can aid in the generation of one-time pads for cryptographic use.

History
An Englishman, Edward Craven Walker, invented the original and best-known lava lamp in the 1960s. He named it the "Astrolight" or "Astro Lamp" and presented it at a Hamburg trade show in 1965, where the entrepreneur Adolph Wertheimer noticed it. Wertheimer and his business partner Hy Spector bought the American rights to the product and began to produce it as the "Lava Lite"&reg; via a corporation called Haggerty Enterprises and trading under the name Lava World International&reg;. The lava lamp became an icon of the 1960s, where the constantly changing, brightly-color display has been compared to the psychedelic hallucinations of certain popular recreational drugs. In the 1990s Mr. Walker sold his rights to Cressida Granger whose company Mathmos continues to make lava lamps and other related products.

Lava lamp Darwin Award
The lamp is immobile until the liquid is sufficiently warmed by a (typically 40-watt) lamp. This can take an hour or longer, especially if it's cold. In a sufficiently cold room, the wax may never start to flow. Because of this, users have been tempted to warm the lamp by other means, which is why the makers include a safety warning. On November 30, 2004, 24-year-old Philip Quinn died in a lava lamp accident. The glass bottle exploded while Mr. Quinn was heating it on top of his home stove, sending a glass shard through his heart, and earning him a Darwin Award.