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Theory:

A direct reading instrument that is used to measure the density of a liquid is a hydrometer. Hydrometer works on the principle of flotation.
  
Flotation principle:
The weight of the liquid displaced by the submerged portion of the hydrometer is equal to the weight of the hydrometer.
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  • A hydrometer is made up of a cylindrical stem having a narrow tube at its upper end and a spherical bulb at its lower end.
  • The lower spherical bulb is partially filled with mercury or lead shots. This arrangement helps the hydrometer to float or stand vertically in liquid.
  • The narrow tube has markings that show the relative density of a liquid.
  • The liquid to be examined is poured into the glass jar.
  • The hydrometer is gently dropped into the liquid until it floats freely.
  • The liquid's relative density can be found by the reading against the level of liquid touching the tube.
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Hydrometer
 
Hydrometers may be modified for different uses, such as saccharometer for measuring the density of sugar in a liquid, lactometers for measuring the density of milk, and alcoholometer for measuring higher alcohol levels in spirits.
Reference:
https://www.flickr.com/photos/taubinphoto/8670718166/