What object did French scientists use to determine the length of a meter?

Originally (around 1871) the length was established by a bar of a certain length, which represented one ten-millionth of the distance from the North Pole to the Equator.

In 1960, it was defined physically by an unchanging wavelength of light: "1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum." This is still a very accurate definition.

Since 1983, it has been defined as "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second." The second is currently defined relative to the radiation emission wavelength of cesium-133, which is a fixed length.