Hydrometer, << hy DROM uh tuhr, >> is an instrument used to determine the density of liquids. The design is based on Archimedes’ principle, which explains buoyancy. The principle states that a body in liquid seems to grow lighter and that the loss of weight is equal to the weight of the displaced liquid.
The hydrometer is usually a glass tube with a weight in the bottom. It is placed in the fluid to be measured and allowed to float. The density of the fluid can be determined by measuring the surface level of the fluid against a scale marked on the side of the tube. When the density of the fluid is divided by the density of water, the result is called the specific gravity of the fluid. If the specific gravity of a liquid is less than one, a hydrometer will sink lower in that liquid than it will in water.
One type of hydrometer, called a lactometer, is used to test the purity of milk. An alcoholometer is used to test alcohol. The strength of brine in the boilers of seagoing ships is tested with a salinometer.
One type of hydrometer is made especially to test storage batteries. It is called an acidimeter, and is used to determine the amount of acid in the batteries. The concentration of battery acid increases the density of the liquid in the battery.
