Future of the kilogram
The kilogram is the last base SI unit that is linked to a physical object; the others are all defined in terms of naturally occurring constants, like the speed of light. By linking the definitions in this way, these units are guaranteed to be stable in the long term thus ensuring that all research and manufacturing will have a consistent standard to work from and all results will be compatible.
Scientists are close to completing two experiments, the Watt balance and the International Avogadro coordination, both of which define the value of the kilogram with relation to a fundamental constant.
The SI unit of mass, the kilogram (kg), is defined as being equal to the mass of the international prototype of the kilogram, the IPK.
The origins of the kilogram date from 1799, when a cylinder of pure platinum was manufactured to have the same mass as one cubic decimetre (equivalent to a litre in today's terms) of water at 4 ºC. The current kilogram replaced this original weight in September 1889 when the first General Conference on Weights and Measures (CGPM) sanctioned the IPK for use as the global standard for the mass. The IPK is a cylinder of 39 mm in height and diameter and is made from an alloy of platinum (90 %) and iridium (10 %).
The IPK is kept at the International Bureau of Weights and Measures (BIPM) in Sèvres just outside Paris. National Measurement Institutes, such as the National Physical Laboratory, possess copies that they use as their national standards. These national standard kilograms are tested against the IPK about every 40 years to see whether their mass has deviated from that of the IPK.
- The IPK would feel 27 times heavier on the Sun than on Earth, 6 times lighter on the Moon, and 12 times lighter on Pluto
- The IPK has a fixed mass but its weight can vary by 1 milligram or more depending on the density of the air in which it is weighed
- New experiments to define the kilogram aim to have a target uncertainty of 2 parts in 108 (0.000 002%)
- By definition, the mass of the IPK is constant; however, at the periodic verifications the mass of the national copies were seen to have changed by up to 50 micrograms. These differences led to the press stories about the 'kilogram losing weight'.
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