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Author
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Topic: Redefining the Kilogram
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DrConway
rabble-rouser
Babbler # 490
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posted 26 April 2005 10:13 AM
Scientists want to bring the kilogram into 21st century quote:
The National Institute of Standards and Technology (NIST) in the United States, is calling for the kilo to be redefined in terms of a natural phenomenon, such as the number of atoms in a silicon crystal, or the amount of magnetic force required to levitate an object in a watt balance.For 115 years, the kilogram has been defined by a lump of platinum-iridium alloy, kept just outside Paris. But scientists argue that relying on an actual physical artifact leave the kilogram vulnerable to misinterpretation, and means other nations have to travel to France to verify their own standard masses, LiveScience reports.
The article goes on to explain that other units we use in everyday life are now defined in terms of measurable quantities that don't depend on the thing you're using to measure it. For example the second used to be defined in terms of Earth's own motion, and the meter used to be defined in terms of a distance. Since both are now defined in terms of the speed of light, which in turn is known to be constant thanks to our knowledge that relativistic effects do indeed take place, we are free of the annoying problem of errors in defining the unit in terms of something that changes over time.  About ten years ago chemists spoke of redefining the kilogram in terms of the mole, 6.02214 * 1023 particles of something, but this foundered, as I understand it, on the fact that redox chemistry does not have the measurement accuracy sufficient to define the kilogram that way. (It's easy enough to work out. Take a known current, which is defined in terms of something unrelated to the kilogram, and pass it through an electrolytic cell. Run for a known time, which is defined in terms of the speed of light, and you will plate out a known mass. That's the theory, anyway. In practice there's too much secondary errors creeping in for the accuracy to work. You have the diffusive behavior of ions in solution, concentration gradients, and so on. You also get current-flow gradients because you're changing the area of the plate which carries the current into solution, which changes the cell constant, and so on.) You can start to get some insight into the difficulty of defining a good basis on which to standardize a unit just from my little exposition in the brackets.
From: You shall not side with the great against the powerless. | Registered: May 2001
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