The common hydrometer was introduced by Robert Boyle in the 17th century. It consists of three parts:
1. The weighed bulb which causes the hydrometer to float upright.
2. The wide bulb which displaces sufficient liquid for the upthrust.
3. The graduated stem which is narrow and graduated to read the specific gravity or density of the liquid in which the hydrometer floats.
Suppose a hydrometer floats in water such that its stem is over the surface. This is marked as 1.0 on the stem as the specific gravity of water is 1. If the hydrometer is floated in a less dense liquid, it will sink lower. Therefore, the marking on the top of the stem will be lower than the 1.0 marking. The narrow the stem, the more will be the sensitivity and accuracy of the hydrometer, as it will rise or fall to a greater extent in liquids of different densities.
Two widely used hydrometers, (i) the lactometer, and (ii) the battery hydrometer, are based on the common hydrometer.