Hydrometer
Hydrometer, an instrument for determining the specific gravity of liquids. A common application is in measuring the concentration of antifreeze in the cooling system of an automobile. The lower the specific gravity of the coolant, the greater is the concentration of antifreeze present. Special forms include the alcoholometer, for measuring the alcoholic strength of a liquid; and the acidimeter, for determining the acid content of a storage battery.
The hydrometer works on the principle that a floating body displaces a volume of liquid whose weight is equal to its own; the lighter the liquid (that is, the less its specific gravity), the deeper the body sinks because a greater amount of liquid is required to equal the body's weight.