The point where the surface of the liquid touches the stem of the hydrometer is noted. Hydrometers usually contain a paper scale inside the stem, so that the specific gravity (or density) can be read directly in gramss per cubic centimeter.
In light liquids like kerosene, gasoline, and alcohol, the hydrometer
must sink deeper to displace its weight of
liquid than in heavy liquids like brine, milk,
and acids. In fact, it is usual to have two
separate instruments, one for heavy liquids,
on which the mark 1.000 for water is near
the top, and one for light liquids, on which
the mark 1.000 is near the bottom of the stem.
Commercial uses
Because the commercial value of many liquids, such as sugar solutions, sulphuric acid, alcohol, and wine, depends directly on the specific gravity, hydrometers are used extensively.
Perhaps the best-known form of hydrometer is the kind used in testing milk, called a lactometer. The specific gravity of cow's milk varies from 1.027 to 1.035. Since only the last two digits are important, the scale of a lactometer runs from 20 to 40, which means from 1.020 to 1.040. The specific gravity of milk does not give us a conclusive test as to its worth. In addition to water (which is about 87%), milk contains some substances that are heavier than water, such as albumen, sugar, and salt, and others that are lighter than water, such as butter fat. So, in addition to the specific gravity, one needs to determine the amount of fat, and, if possible, of other solids in the milk, to know its richness. Of course, the very important question as to the cleanliness of milk must be left to the bacteriologist.
