Instrument used for measuring the density or relative density of liquids is known as

Density

1. Density is the amount of matter in unit volume.
2. The density of a matter is defined by the form, $\rho =\frac{M}{V}$, where M and V are the mass and volume of the body.

Relative density

1. Relative density is the density of a matter with respect to another matter.
2. The Relative density of a water with respect to gas is defined by the form, ${\rho }_r=\frac{{\rho }_w}{{\rho }_g}$, where ${\rho }_w$ and ${\rho }_g$ are the density of water and gas.

Instrument to measure density and relative density

1. Hydrometer is the instrument to measure density and relative density.
2. Hydrometer was invented and developed by William Nicholson in 1790.

So, Hydrometer is used for measuring the density or relative density of liquids.