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MEASUREMENT
Meter
  Meter is the unit of length in S.I. System.
  Meter is defined as "The distance between the two marks on a Platinum-Iridium bar kept at 0OC in the   International Bureau of Weight and Measures in Paris."
  One meter = 100 cm
  One meter = 1000 mm
Kilogram
  Kilogram is the unit of mass in S.I. System.
  "Kilogram is defined as the mass of a platinum cylinder placed in the International Bureau of Weight and   Measures in Paris."
  One kilogram = 1000gram
Second
  Second is the unit of time in S.I. System.
  A second is defined in terms of the time period of Cs-133 atoms.
  i.e." one second is equal to 9,192,631,770 periods of vibrations of Cs-133 atoms."
  60 seconds = one minute
  3600 seconds = one hour
Least Count
  Minimum measurement that can be made by a measuring device is known as " LEAST COUNT'.
  Least count (vernier callipers) = minimum measurement on main scale / total number of divisions on                                                                                                               vernier scale
.
  Least count (screw gauge) = minimum measurement on main scale / total number of divisions on                                                                                                    circular scale
  Smaller is the magnitude of least count of a measuring instrument, more precise the measuring   instrument is.
  A measuring instrument can not measure any thing whose dimensions are less than the magnitude of   least count.
  Least Count of Vernier Callipers = 0.01 cm
  Least Count of Micrometer Screw gauge = 0.001 cm
Zero Error
  It is a defect in a measuring device (Vernier Callipers & Screw Gauge).
  When jaws of a Vernier Callipers or Screw Gauge are closed, zero of main scale must coincides with the   zero of vernier scale or circular scale in case of screw gauge.
  If they do not coincide then it is said that a zero error is present in the instrument.
Types Of Zero Error
  Zero error may be positive or negative.
  A positive zero error in the instrument shows a larger measurement than the actual measurement.
  In order to get exact measurement, positive zero error is subtracted from the total reading.
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  A negative zero error in the instrument shows a smaller measurement than the actual measurement.
  In order to get exact measurement, negative zero error is added to the total reading.
Pitch
"Perpendicular distance between two consecutive threads
of the screw gauge or spherometer is called PITCH."
Pitch = Distance traveled on main scale / total number of rotations
Error
An error is defined as
"The difference between the measured value and actual value."
  If two persons use the same instrument for measurement for finding the same measurement, it is not   essential that they may get the same results. There may arises a difference between their   measurements. This difference is referred to as an "ERROR".
Types Of Error
  Errors can be divided into three categories:
  (1) Personal Error
  (2) Systematic Error
  (3) Random Error
Personal Error
  An error comes into play because of faulty procedure adopted by by the observer is called "PERSONAL   ERROR".
  Personal error comes into existence due to making an error in reading a scale. It is due to faulty   procedure adopted by the person making measurement.
Systematic Error
  The type of error arises due to defect in the measuring device is known as "SYSTEMATIC ERROR".
  Generally it is called "ZERO ERROR". it may be positive or negative error. Systematic error can be   removed by correcting measurement device.
Random Error
  The error produced due to sudden change in experimental conditions is called "RANDOM ERROR".
  For example:
  Sudden change in temperature, change in humidity, fluctuation in potential difference (voltage).
  It is an accidental error and is beyond the control of the person making measurement.
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