Gas station without pumps

2012 May 16

The kilogram

Filed under: home school — gasstationwithoutpumps @ 21:42
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IEEE Spectrum has just published a nice article about the definition of the kilogram: The Kilogram, Reinvented.  The article starts with the current standard (a hunk of platinum iridium alloy), which appears to have drifted at least 50 parts per million over the past century, and discusses some of the approaches being looked at to replace the hunk of metal with a more fundamental unit: most likely Planck’s constant.

Note that the meter is defined in terms of the speed of light and the second, and the second in terms of a particular quantum-mechanical transition, so both are tied to fundamental physical constants.


The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
It follows that the speed of light in vacuum is exactly 299 792 458 metres per second, c0 =299 792 458 m/s. [SI brochure, Section]


The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.
This definition refers to a caesium atom at rest at a temperature of 0 K. [SI brochure, Section]

There are two competing methods vying to become the next definition of the kilogram: one based on measuring Planck’s constant with a watt balance, the other based on counting the number of silicon atoms in a sphere of isotopically pure silicon.  Currently, the different methods don’t agree with each other well enough to be accepted as the next standard (though they are already arguably better than the hunk-of-metal standard).

The article is well worth reading for those interested in how measurement standards are created.

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