New def­i­ni­tion for kilo­gram kicks in Mon­day

The Charlotte Observer (Sunday) - - News - BY DEB­O­RAH NETBURN Los An­ge­les Times

In a sub­ter­ranean vault in a sub­urb of Paris lies a small, rarely seen metal cylin­der known as Le Grand K.

For 130 years, this golf­ball-sized hunk of 90% platinum and 10% irid­ium has served as the in­ter­na­tional pro­to­type kilo­gram. That means it was the sin­gle phys­i­cal ob­ject by which all other kilo­grams across the planet were mea­sured.

If mi­cro­scopic con­tam­i­nants in the air caused Le Grand K to grow a bit heav­ier, the kilo­gram it­self grew a bit heav­ier. If a rig­or­ous clean­ing or small scratch caused it to be­come ever so slightly lighter, the kilo­gram it­self be­came lighter as well. In­deed, it is es­ti­mated that over the course of its life­time, Le Grand K has lost 50 mi­cro­grams of mass.

But the long reign of Le Grand K is about to come to an end.

Start­ing Mon­day, the kilo­gram will be re­de­fined not by an­other ob­ject, but by a fun­da­men­tal prop­erty of na­ture known as Planck’s con­stant. Like the speed of light, the value of Planck’s con­stant can­not fluc­tu­ate – it is built with ex­quis­ite pre­ci­sion into the very fab­ric of the uni­verse.

“Un­like a phys­i­cal ob­ject, a fun­da­men­tal con­stant doesn’t change,” said Stephan Sch­lam­minger, a physi­cist at the Na­tional In­sti­tute of Stan­dards and Tech­nol­ogy (NIST) in Gaithers­burg, Md. “Now a kilo­gram will have the same mass whether you are on Earth, on Mars or in the An­dromeda galaxy.”

Re­searchers who have de­voted their lives to the sci­ence of mea­sure­ment say the new def­i­ni­tion of the kilo­gram – and sim­i­lar changes to the mole (which mea­sures quan­ti­ties of very small par­ti­cles), the am­pere (which mea­sures elec­tri­cal charge) and the kelvin (which mea­sures tem­per­a­ture) – rep­re­sents a pro­found turn­ing point for hu­man­ity.

“The abil­ity to mea­sure with in­creas­ing ac­cu­racy is part of the ad­vance­ment of our species,” said Wal­ter Copan, di­rec­tor of NIST.

Most of us reg­u­lar folks will hardly no­tice the switch. A 4-pound chicken (1.81437 kilo­grams) at the grocery store or a pound of cof­fee beans (0.453592 kg) at Star­bucks will re­main ex­actly the same.

“We don’t want to shock the sys­tem,” Sch­lam­minger said.

The de­ci­sion to re­de­fine four base units of the In­ter­na­tional Sys­tem of Units was made in No­vem­ber at the 26th Gen­eral Con­fer­ence on Weights and Mea­sures in Ver­sailles, France. Del­e­gates from 60 mem­ber states as­sem­bled in a large au­di­to­rium for the his­toric vote. It was unan­i­mous. A stand­ing ova­tion and cham­pagne toast fol­lowed.

“The meet­ing it­self was an electric ex­pe­ri­ence,” said Copan, who rep­re­sented the U.S. “It was a long jour­ney to get to this point.”

The ori­gins of the met­ric sys­tem date back to the French Rev­o­lu­tion in the late 1700s. At the time, an es­ti­mated 250,000 dif­fer­ent units of mea­sure­ment were be­ing used in France, mak­ing com­merce and trade a chal­lenge. The new sys­tem was de­signed to be ra­tio­nal and uni­ver­sal, with units based on prop­er­ties of na­ture rather than royal de­cree or the whims of lo­cal dukes and mag­is­trates.

“The idea was that these mea­sure­ments would be eter­nal and the same for ev­ery­body, ev­ery­where,” said Ken Alder, a sci­ence his­to­rian at North­west­ern Univer­sity in Evanston, Ill.

The foun­da­tional unit of the sys­tem was the me­ter, which was sup­posed to be one ten-mil­lionth the dis­tance from the North Pole to the equa­tor along the Paris merid­ian.

At the same time, the kilo­gram was de­fined as the mass of 10 cu­bic cen­time­ters of wa­ter at 4 de­grees Cel­sius.

These units were adopted by the French Re­pub­lic in 1795. Coun­tries in Europe and South Amer­ica adopted the met­ric sys­tem through­out the 19th cen­tury. In 1875, del­e­gates from the U.S. and 16 other coun­tries signed the Treaty of the Me­ter in Paris. It es­tab­lished a uni­ver­sal sys­tem of units based on the me­ter, the kilo­gram and the sec­ond that would stream­line trade among na­tions.

Al­though the me­ter and the kilo­gram were based on the size of Earth, they were of­fi­cially de­fined by metal ar­ti­facts, in­clud­ing Le Grand K, that were cast in Lon­don in 1889 and kept in a vault in the base­ment of the newly cre­ated In­ter­na­tional Bureau of Weights and Mea­sures in Sevres, France. Mem­ber na­tions re­ceived one of 40 pre­cise repli­cas.

The Treaty of the Me­ter also es­tab­lished the Gen­eral Con­fer­ence on Weights and Mea­sures (CGPM), an in­ter­na­tional group tasked with study­ing and vot­ing on pro­posed changes to units of mea­sure­ment for all mem­ber states.

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