乱伦强奸

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The kilogram is the last remaining measurement to be defined by a physical object: a cylinder of platinum and iridium held in a vault under three glass bell jars at the International Bureau of Weights and Measures (BIPM) in the suburbs of Paris. The international prototype, created in the 1880s and known familiarly as 鈥淟e Grand K,鈥� is the standard by which all other kilograms are measured.

But for all the vault-like protections, Le Grand K is vulnerable.

鈥淭he big joke is, if someone were to sneeze on the kilogram, there are about 10 fundamental constants that would change, because they鈥檙e all tied to its value,鈥� says Jonathan Ellis, an assistant professor of optics and mechanical engineering and a specialist in metrology, or the study of measurements.

And it鈥檚 not just a joke. Compared to official replicas held by other laboratories around the globe, Le Grand K is shrinking, albeit by just .05 milligrams in mass.

Established during the French Revolution by the French Academy of Sciences to standardize measurement, the seven basic units of measurement鈥攖he kilogram, the meter, the second, the ampere, the mole, the candela, and the kelvin鈥攁re now, with the exception of the kilogram, determined by what are called 鈥渞ealized standards,鈥� or procedure-based methods. 鈥淚nstead of being tied to an artifact, you鈥檙e tied to how you perform the measurement,鈥� Ellis says.

He and the members of his 乱伦强奸 lab are involved in an effort to create such a procedure for the kilogram. They鈥檙e contributors to a larger project being carried out by a group of national laboratories, including the BIPM and others from countries such as Denmark, Japan, Germany, and Australia. The lab representing the United States is the National Institute for Standards and Technology.

One effort to define the kilogram in terms of natural properties, begun in 1999, uses the watt balance, a device that defines mass by equating electrical and mechanical power. Ellis focuses his lab on how to make velocity measurements more accurate.

鈥淲hen you use light to measure things, one of the limiting factors is the air,鈥� he says. 鈥淟ight has to pass through the air, and air has turbulence.鈥� There are fluctuations in air pressure and in temperature. 鈥淔or example, the human body will output about 100 watts of power, and so there will be a temperature gradient because you鈥檙e standing near the instrument, and that will affect the light that passes through. We鈥檙e talking about very, very, very small features and small distances鈥濃�攐n the atomic scale鈥斺�渁nd so very small perturbations can cause a significant problem.鈥� Even someone talking in the lab creates disturbance, with sound pressure and the exhalation of carbon dioxide.

So Ellis is taking part in an effort to find a way to measure that鈥攁nd to do so more quickly than a temperature sensor or a pressure sensor can manage.

The watt balance method isn鈥檛 the only one under investigation. There is a competing method, called Avogadro鈥檚 Sphere, in which scientists are trying to make a pure silicon sphere that has the number of atoms that would equal the mass of one kilogram.

鈥淏oth measurements in and of themselves are accurate, but they don鈥檛 conform to each other,鈥� Ellis says. 鈥淭here鈥檚 some discrepancy there, and the international community is trying to figure out where, why, and how.鈥�

鈥擪athleen McGarvey锘匡豢