Vernier Calliper

• Measurement


  The process of determining the size, amount, or degree of something, using an instrument
• Instruments used for measurement

  • Metre rule (or vernier callipers) for length
  • Beam balance for mass
  • Watch for time
  • Thermometer for temperature
  • Ammeter for current
• Least count


  The smallest measurement that can be taken accurately with an instrument
• Measuring instrument

  • Provided with a graduated scale for measurement
  • Least count is the value of one smallest division on its scale
• Principle of Vernier

  1. Two scales used (main scale and vernier scale)

  2. Main scale is fixed, vernier scale slides along it

  3. Vernier scale has n divisions equal to (n-1) divisions of main scale

  4. Least count = value of 1 main scale division / total divisions on vernier
• Vernier constant or least count of vernier


  Difference between the values of one main scale division and one vernier scale division
• Vernier callipers

  • Used to measure length of rod, diameter of sphere, internal/external diameter of cylinder, depth of beaker
  • Consists of main scale, vernier scale, jaws, screw to fix vernier, strip to measure depth
• Zero error in vernier callipers

  • Mechanical error where zero mark of vernier does not coincide with zero mark of main scale when jaws are in contact
  • Positive zero error - zero mark of vernier right of main scale
  • Negative zero error - zero mark of vernier left of main scale
• Pitch of a screw


  Distance moved along axis by screw in one complete rotation of its head
• Least count of a screw

  If pitch is 1mm and head has 100 divisions, then 1 division of head scale = 0.01mm
• Screw


  An ordinary screw has threads on it at an equal distance along its length. On rotating the head of the screw, it moves forward or backward linearly along its axis.
• Pitch of a screw


  The distance moved along its axis by the screw in one complete rotation of its head
• Generally, the pitch of a screw is 1 mm or 0.5 mm
• Circular/head scale


  The head of the screw is graduated along its circumference, normally with 50 or 100 equal divisions
• Least count of a screw


  The distance moved by the screw along its axis, on rotation of 1 division of the circular scale
• Least count of a screw

  • Pitch = 1 mm, Divisions = 100, Least count = 0.01 mm

  1. Pitch = 0.5 mm, Divisions = 50, Least count = 0.01 mm
  2. Pitch = 0.5 mm, Divisions = 500, Least count = 0.001 mm
• Screw gauge


  A device that works on the principle of a screw, used to measure the diameter of a wire or thickness of a paper, usually up to an accuracy of third decimal place of a cm (0.001 cm)
• Screw gauge

  • U-shaped frame with a flat end (stud) at one end and a nut with a cylindrical sleeve at the other end

  Screw with a flat end that can be moved inside the nut by rotating its head (thimble)

  Main scale graduated in mm on the cylindrical sleeve

  Circular scale (head scale) divided into 100 equal parts on the thimble

  Ratchet mechanism to hold the object gently between the stud and the screw end
• Pitch of a screw gauge

  The linear distance moved by its screw on the main scale when the circular scale is given one complete rotation
• To decrease the least count of a screw gauge: Decrease the pitch and increase the total number of divisions on the circular scale
• A micrometer screw gauge has 20 divisions in 1 cm on main scale and 500 divisions on its circular scale, with a pitch of 0.5 mm and a least count of 0.001 mm
• Zero error in a screw gauge


  When the zero mark of circular scale does not coincide with the base line of main scale on bringing the flat end of the screw in contact with the stud
• Positive zero error

  Zero mark on the circular scale is below the base line of main scale
• Negative zero error

  Zero mark on the circular scale is above the base line of main scale
• Backlash error

  Error caused by wear and tear of screw threads, where the tip of the screw does not start moving in the opposite direction immediately on reversing the rotation of the thimble
• To avoid backlash error, rotate the screw in one direction only. If changing direction, move further in the same direction, pause, then rotate in the opposite direction.