Theory:

A lactometer is a type of hydrometer. It is a device that measures the purity of milk, and it operates based on the gravity of milk.
  
Construction:
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Lactometer
  • The lactometer comprises a long calibrated test tube with a cylindrical bulb with a graduation of \(15\) \(cm\) at the top and \(45\) \(cm\) at the bottom.
  • The test tube is completely filled with air. This air chamber causes the device to float when it is placed in the milk.
  • In addition, the spherical portion of the bulb is filled with mercury to make the lactometer to sink up to the proper level and float in an upward position in milk.
  • A thermometer is placed inside the lactometer, which extends from the bulb into the upper portion of the test tube where the scale is located.
  • A proper lactometer reading be achieved only at a temperature of \(60\) degrees Fahrenheit. A lactometer is an instrument that measures the amount of cream present in milk.
  • The lower the lactometer floats in the milk, the more cream there is.
  • Quality milk has an average reading of \(32\).
  • Lactometers are used in the milk production process.
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Testing the purity of milk
Reference:
https://www.flickr.com/photos/ilri/13889870345