An international prototype metre bar was made of a 90-10 platinum-iridium alloy with an X cross section to minimize flexing. The metre was defined as the distance measured between two lines on the bar at the temperature of freezing water, 0 °C. The alloy ingot was made possible by a high-temperature furnace devised by Henry Sainte-Claire and his assistant Jules Henry Debray.
The alloy was harder than pure platinum, but retained the other desireable properties, including resistance to corrosion. Duplicate prototypes were made and distributed to each of the countries that joined in the international agreement. The same alloy was used to prepare the international protype of the kilogram.
According to todayinsci