Ludions

Ludions, also known as Cartesian divers, are self-contained toys or scientific demonstrations consisting of a partially filled, often hollow, inverted figurine or object placed inside a closed, liquid-filled container. The device demonstrates principles of buoyancy, pressure, and Archimedes' principle.

Mechanism:

The ludion functions based on the compressibility of air. The hollow figurine contains a small amount of air trapped within it. When pressure is applied to the flexible walls of the container, the pressure is transmitted through the liquid and also increases the pressure on the air bubble trapped inside the ludion.

This increase in pressure compresses the air bubble. As the air bubble shrinks in volume, it displaces less water. Because the density of water is higher than the density of air, compressing the air increases the overall density of the ludion.

If the density of the ludion increases to the point where it is greater than the density of the surrounding liquid, the ludion loses buoyancy and sinks. Conversely, when the pressure on the container is released, the air bubble expands, the ludion becomes less dense, and it rises.

Construction:

Ludions are typically constructed using a small, hollow figurine or object (often made of glass or plastic), such as a diver, a devil, or a fish. The figurine is weighted so that it floats just barely at the surface of the water when placed in the container. An airtight, flexible container (often a plastic bottle) is then filled completely with water. The figurine is placed inside, and the container is sealed.

Historical Significance:

The Cartesian diver is named after René Descartes, the 17th-century philosopher and scientist. While Descartes did not invent the device, its principles align with his work on fluid dynamics and pressure. The ludion has been used for centuries as a demonstration of scientific principles and as a simple, engaging toy.