 Hydrometer

A floating density instrument. At equilibrium, Archimedes' principle gives $\rho V_{\mathrm{displaced}}=m$ for fixed instrument mass $m$, so it displaces more volume in a less dense liquid. A graduated narrow stem converts this volume change into a visible immersion reading. Calibration specifies a temperature and a reading convention for the meniscus; a sample's other dissolved substances can affect a sugar-calibrated scale.