The Invar substandard metre is contained within its own box and has a cross-section which looks like: H The bar is marked off in millimetres, with every tenth one enumerated. The last (1000th) millimetre has ten fine subdivisions. A travelling microscope would have been used to make measurements with the substandard metre. Preserved in each country was a standard metre against which the substandards were compared.
Invar is a nickel-steel alloy (35.6% nickel) with an extremely low coefficient of thermal expansion. It was invented in 1890 by the Swiss Charles-Edouard Guillaume (1861-1938) whilst working for the International Bureau of Weights and measures in Paris. Guillaume received the Nobel Prize for Physics in 1920 for his work with nickel-steel alloys.
The metric system came into being in France in the 1790s. The metre was originally defined as 10-7 of the Earth's quadrant (passing through Paris) and the first standards were platinum. During the nineteenth century, the metric system was gradually embraced by nations world-wide. In 1960 the metre was redefined in terms of the wavelength of the orange-red line in the krypton-86 spectrum and more recently as the distance travelled by light in vacuum in a certain number of oscillations of a Cs clock.
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The thermal coefficient of expansion for Invar is about 1.2 ppm per degC. Used for pendulums. Although it has a low thermal coefficient of expansion, it is subject to creep.
