What tool is used to measure relative density?

The tool used to measure relative density is a hydrometer. A hydrometer works by floating in a liquid and providing a reading based on the depth it sinks. The principle behind its function is based on Archimedes' principle, which states that a body immersed in a fluid experiences a buoyant force equal to the weight of the fluid displaced. The more dense a liquid is, the higher the hydrometer will float, allowing for the measurement of the liquid's specific gravity or relative density compared to water.

In contrast, a barometer is specifically designed to measure atmospheric pressure, while a ruler is a tool for measuring length, and a manometer is used for measuring the pressure of fluids, particularly gases. These instruments serve distinct purposes that do not relate to measuring the density of liquids, highlighting why the hydrometer is the correct choice for determining relative density.