Lactometer is an instrument for testing the quality of milk. One form of lactometer is a hydrometer to which is attached a scale about five inches long, the zero of which marks the point to which it just sinks in water. Another point on the scale corresponds to average quality milk, and according as this mark sinks below or rises above the level of the milk, the milk is said to be more or less diluted with water or the richness of milk.
Put some milk in the test-tube or long glass.
Dip the meter bulb in it, the bulb going in first down.
You will notice that the meter bulb floats.
The reading on the meter indicates how pure / impure your milk is.