Iridium melting point full#

The addition of 10 % iridium to the platinum greatly increases its hardness and so reduces wear. Platinum and its alloys are reasonably easy to machine, enabling a good surface finish to be achieved on the artefact, again reducing the effect of surface contamination. Minimizing the volume of the weight minimizes the effect of changing air density on the weight of the artefact. The weight-in-air of the kilogram (or any mass standard) depends on the density of the air in which it is weighed because the air (or any fluid in which it is weighed) exerts a buoyancy effect proportional to the volume of the artefact. The high density of the material also means that it displaces a smaller amount of air than a kilogram of less dense material (stainless steel or brass for example). The relatively inert nature of the material also minimizes surface contamination and enhances the mass stability of the artefact. m в€’3) means that the artefact has a small surface area and, therefore, the potential for surface contamination is minimized.Platinum-iridium was chosen as the material for the kilogram for a number of reasons. The design of the artefact minimizes its surface area while making it easy to handle and machine (a sphere would give the minimum surface area but presents difficulties in manufacture and use). The International Prototype Kilogram (commonly known as the (International) Kilogram or just K) is a cylinder of approximate dimensions 39 mm diameter by 39 mm height (see Figure 2.4). Twenty of these were allocated to the signatories of the convention of the metre as national standards. Forty further kilogram weights were produced using the same techniques and delivered in 1884. One of these kilograms was designated K and became the International Prototype Kilogram. Three cylindrical pieces were delivered to St-Claire Deville metallurgists in France where they were hammered in a press to eliminate voids, rough machined and polished and finally adjusted against the kilogram des archives. Leach, in Fundamental Principles of Engineering Nanometrology, 2010 10.1.1 Manufacture of the Kilogram weight and the original copiesДЄfter many attempts in France, Johnson Matthey of London made a successful casting of a 90 % platinum 10 % iridium alloy mass standard in 1879.

Iridium melting point full#

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