Measurement of a quantity is the act or result of a quantitative comparison between a predefined standard and an unknown magnitude. The act of measurement must satisfy,
a) The standard must be accurately known and internationally accepted.
b) The apparatus and the experimental procedure adopted for comparison must be provable.
The standard of length is the standard meter defined as a
length between two very fine lines engraved on a platinum-iridium bar of X cross section maintained and measured under very accurately controlled conditions. According to the General Conference of Weight and Measures, the standard meter is equivalent to 1,650,763.73 wavelengths of Kr86 orange-red light.
The two basic methods of measurements are:
a) A direct comparison with the primary or the secondary standard and
b) An indirect comparison with a standard, through a calibrated system
Calibration:
Calibration procedures involve a comparison of a particular instrument with either
(i) a primary standard, or
(ii) a secondary standard, or
(iii) a known input source.
The secondary standard employed for calibration must possess accuracy at least ten times higher than that of the instrument to be calibrated.
