Question

what is lactometer and what is the principle of working of lactometer

Answer 1

Lactometer is a small glass instruments used to test the purity of the milk. It is used to find out the amount of water in the milk. Works on the principle of the specific gravity of milk. (Archimede's principle)

Answer 2

Lactometer is a device used for testing the purity of milk.

It measures relative density of milk with respect to water, which is also called specific gravity. 

If the specific gravity of a sample of milk is within the approved ranges, the milk is pure.

If it is not, then there is some adulteration.