Meter

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.