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(high-DRAH-mih-ter)
Literally meaning "water measurer," a
hydrometer is an instrument comprised of a vertical scale inside a sealed glass
tube weighted at one end. It's used to measure the ratio (called specific gravity) of the density
of a liquid (such as sulfuric acid)
to that of pure water. A hydrometer floats upright in liquid; the reading
is taken where the liquid's surface hits the scale-and the hydrometer
floats higher in denser liquid.
Commercial hydrometers are usually calibrated for
ordinary room temperature, which is taken to be 20°C (68°F). Because of the
variation in the depth to which the instrument sinks in heavy and in light
liquids, one type is made for use in liquids more dense than water and
another for use in those less dense than water.
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