To measure the density of a liquid.
Hydrometer, one-liter graduated cylinders with water and saline solution.
If the hydrometer is immersed in a graduated cylinder filled with water, the reading on the scale (at right in picture) should be C(W) = 1000, indicating that the scale is calibrated for water. If the hydrometer is put into the saline solution, the number C(L) at the level of the liquid is read. The density D(L) of the unknown liquid is given by: D(L)=C(L)*D(W)/1000,where D(W) is the density of water in kg/m^3. One can measure densities between 0.7 and 2.0 using this device. ◙