Hydrometer
= Hydrometer
{wiki}
A floating <density> instrument. At equilibrium, <Archimedes' principle> gives $\rho V_{\rm displaced}=m$ 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.