What is a hydrometer? Name the two categories of hydrometers and explain the principle on which its working is based.

Hygrometer is an instrument which is used to measure the relative density of fluid.
Following are the two categories of hygrometer
Lactometer: Instrument which is used to measure the density of milk or purity of milk.
Alcoholometer:-It is use to measure the density of alcohol in spirits.
working principle:-
It works on Archimedes principle. When a solid body is suspended in then it is buoyed by a force equal to the weight of the fluid displaced by the submersed part of the solid.
If the density of the fluid is lower the hygrometer will sink deeper.