International Prototype of the Kilogram

The International Prototype of the Kilogram (referred to by <a href="/facts/Metrology/HC0MmLdb">metrologists</a> as the IPK or Le Grand K; sometimes called the ur-kilogram, or urkilogram, particularly by German-language authors writing in English:<a href="https://books.google.com/books?id=MtgoDwAAQBAJ&dq=%22the%20ur-kilogram%20in%20Paris%22&pg=PA30">30</a>: 64 ) is an object whose mass was used to define the <a href="/facts/Kilogram/lU2zbPk8">kilogram</a> from 1889, when it replaced the <a href="/facts/Kilogramme_des_Archives/hGc1eMad">Kilogramme des Archives</a>, until 2019, when it was replaced by a <a href="/facts/2019_revision_of_the_SI/g0h07jCX">new definition of the kilogram</a> based entirely on <a href="/facts/Physical_constant/uI2QvAHw">physical constants</a>. During that time, the IPK and its duplicates were used to calibrate all other kilogram mass standards on Earth.
The IPK is a roughly golfball-sized object made of a <a href="/facts/Platinum%E2%80%93iridium_alloy/6N1WVG1u">platinum–iridium alloy</a> known as "Pt‑10Ir", which is 90% <a href="/facts/Platinum/qJC6lQf7">platinum</a> and 10% <a href="/facts/Iridium/TFQutoj1">iridium</a> (by mass) and is machined into a <a href="/facts/Right-circular_cylinder/0wQPPr3k">right-circular cylinder</a> with perpendicular height equal to its diameter of about 39 <a href="/facts/Millimetre/25mdTXpa">millimetres</a> to reduce its surface area. The addition of 10% iridium improved upon the all-platinum Kilogramme des Archives by greatly increasing <a href="/facts/Hardness/PtGn6E20">hardness</a> while still retaining platinum's many virtues: extreme resistance to <a href="/facts/Redox/Pyd5RVcj">oxidation</a>, extremely high <a href="/facts/Density/92p6hh9I">density</a> (almost twice as dense as <a href="/facts/Lead/EYljasJN">lead</a> and more than 21 times as dense as <a href="/facts/Properties_of_water/FFsl65l1">water</a>), satisfactory <a href="/facts/Electrical_conductivity/akyCKvMy">electrical</a> and <a href="/facts/Thermal_conductivity/qWnSG3nQ">thermal conductivities</a>, and low <a href="/facts/Magnetic_susceptibility/hWMbGTjP">magnetic susceptibility</a>. 
By 2018, the IPK underpinned the definitions of four of the seven <a href="/facts/SI_base_units/M7PsTWg4">SI base units</a>: the kilogram itself, plus the <a href="/facts/Mole_(unit)/kCMFwCc0">mole</a>, <a href="/facts/Ampere/MScqkCgz">ampere</a>, and <a href="/facts/Candela/grEDstnw">candela</a> (whose definitions at the time referenced the <a href="/facts/Gram/sRSW5RJb">gram</a>, <a href="/facts/Newton_(unit)/Ral9XgNl">newton</a>, and <a href="/facts/Watt/VK8oqRrm">watt</a> respectively) as well as the definitions of every named <a href="/facts/SI_derived_unit/RLu52lWy">SI derived unit</a> except the <a href="/facts/Hertz/6Wshj5qm">hertz</a>, <a href="/facts/Becquerel/4C3yrsqp">becquerel</a>, <a href="/facts/Degree_Celsius/okAEu1CA">degree Celsius</a>, <a href="/facts/Gray_(unit)/GFDaGPSL">gray</a>, <a href="/facts/Sievert/8HLEH1vG">sievert</a>, <a href="/facts/Farad/gPAYcVoE">farad</a>, <a href="/facts/Ohm_(unit)/QNVrlGjw">ohm</a>, <a href="/facts/Siemens_(unit)/kLUHhcCj">siemens</a>, <a href="/facts/Henry_(unit)/uwVv9zue">henry</a>, <a href="/facts/Radian/Srd38Sxa">radian</a> and <a href="/facts/Steradian/PRCqT4n1">steradian</a>.
The IPK and its six sister copies are stored at the <a href="/facts/International_Bureau_of_Weights_and_Measures/tPEu4X0s">International Bureau of Weights and Measures</a> (known by its French-language initials BIPM) in an environmentally monitored safe in the lower vault located in the basement of the BIPM's <a href="/facts/Pavillon_de_Breteuil/UWvsCBkV">Pavillon de Breteuil</a> in <a href="/facts/Saint-Cloud/tUDkkDWF">Saint-Cloud</a> on the outskirts of Paris (see External images, below, for photographs). Three independently controlled keys are required to open the vault. Official copies of the IPK were made available to other nations to serve as their national standards. These were compared to the IPK roughly every 40 years, thereby providing <a href="/facts/Traceability/g0GteSaq">traceability</a> of local measurements back to the IPK.

International Prototype of the Kilogram open-in-new

International Prototype of the Kilogram