Question

which instrument is used to determine the purity of milk

Answer 1

The name of the device used to determine the purity of milk is Lactometer. Lactometer checks the purity of milk by measuring its density. The instrument under discussion operates on the principle of the specific gravity of milk. Thus, by working on the basis of the Archimede’s principle the lactometer tends to float on the surface of milk if it is pure whereas in case of any adulteration it sinks in the milk.  

Answer 2

It is the lactometer which is used to determine the purity of the milk.  

The milk is poured into the container and is allowed to stand until the cream has formed. it is the depth of the formation of the cream which determines the milk's purity.

If it pure, the lactometer floats or otherwise sinks.