A hydrometer is usually made of glass and consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upright. 
The liquid to be tested is poured into a tall container, often a graduated cylinder, 
and the hydrometer is gently lowered into the liquid until it floats freely. The point at which the surface of the liquid touches the stem of the hydrometer is noted. 
Hydrometers usually contain a scale inside the stem, 
so that the specific gravity can be read directly. A variety of scales exist, and are used depending on the context.

Hydrometers may be calibrated for different uses, such as a lactometer for measuring the density (creaminess) of milk, 
a saccharometer for measuring the density of sugar in a liquid, or an alcoholometer for measuring higher levels of alcohol in spirits.

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A hydrometer is used to test the state of charge of a battery cell.
 This is performed by measuring the density of the electrolyte, 
which is accomplished by measuring the specific gravity of the electrolyte. The greater the concentration of sulfuric acid,
 the more dense the electrolyte becomes. 
The higher the density, the higher the state of charge.