# This Day in History (28-Sep-1889) – The length of a meter is defined by the General Conference on Weights and Measures

The origins of the meter go back to at least the 18th century, when there were two approaches to the definition of a standard unit of length. One defining the meter as the length of a pendulum having a half-period of one second; others suggested defining the meter as one ten-millionth of the length of the earth’s meridian along a quadrant (one fourth the circumference of the earth). In 1791, soon after the French Revolution, the French Academy of Sciences chose the meridian definition over the pendulum definition because the force of gravity varies slightly over the surface of the earth, affecting the period of the pendulum. Thus, the meter was intended to equal one ten-millionth of the length of the meridian through Paris from pole to the equator. However, the first prototype was short by 0.2 millimeters because researchers miscalculated the flattening of the earth due to its rotation. Still this length became the standard.

In 1889, a new international prototype was made of an alloy of platinum with 10 percent iridium, to within 0.0001, that was to be measured at the melting point of ice.  The prototype of the meter was sanctioned by the 1st CGPM – The General Conference on Weights and Measures (Conférence Générale des Poids et Mesures) in 1889, and is still kept at the BIPM (Bureau International des Poids et Mesures) under the conditions specified. In 1927, the meter was more precisely defined as the distance, at 0°, between the axes of the two central lines marked on the above mentioned bar, this bar being subject to standard atmospheric pressure and supported on two cylinders of at least one centimeter diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other.

The 1889 definition of the meter, was replaced by the CGPM in 1960 using a definition based upon a wavelength of krypton-86 radiation. This definition was adopted in order to reduce the uncertainty with which the meter may be realized. In turn, to further reduce the uncertainty, in 1983 the CGPM replaced this latter definition by the following definition: The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Reference:

http://www.mapsofworld.com/on-this-day/september-28-1066-ce-the-norman-conquest-of-england-begins

http://physics.nist.gov/cuu/Units/meter.html