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Definition of the meter has gradually changed. Chronologically, the following four definitions can be considered for the most interesting.
**Pendulum meter**
The length of a pendulum, which half cycle (a swing ‒ half oscillation) takes one second.
**Meridional meter**
Meridional meter was defined as one ten-millionth of the distance between the North pole and the equator along the Paris meridian. Circumference of the Earth therefore amounted to exactly 40,000 kilometers.
**Prototype meter bar**
Prototype meter bar is standard international prototype of meter which was defined as the distance between two points of standard bar made of the alloy of 90% platinum and 10% iridium measured at the melting point of ice.
**Wave meter**
The meter is the length of the path traveled by light in vacuum during a time interval of 1/299792458 of a second.
