What is a hydrometer?

A hydrometer is a device used to specify a liquid's density. It is typically made of glass with a stem protruding from a bulb filled with mercury or lead that serves as a weight.

The stem of a hydrometer is marked with specific numbers in order to determine the density of a liquid. When it is placed in a liquid, the mercury or lead-filled bulb weighs down the hydrometer, allowing the stem to be immersed. The buoyancy of the hydrometer increases with higher density, while it decreases with lower density and sinks deeper into the liquid. The numbers on the stem mark the hydrometer's depth in the liquid, determining the density of the liquid.