b'\x1b\x1a\x19 \x1f\x18\x1d\x17\x16Metric MattersDEVELOPMENT OF KILOGRAM AND SECONDBy Edmund Isakov C (39.2 F), became a little greaterof the cesium-133 atom at the Na-than 1 dm 3(61.02 cu. in.), where 1 Ltional Physical Laboratory in Lon-K ing of France Louis XVI com- equals 1.000028 dm 3 . don. The clock, accurate to one sec-missioned a group of scientists,On Oct. 13, 1964, the 12th CGPMond in 300 years, was sufficient to including the prominent Antoinemeeting authorized the equality: 1 Ldetect irregularities in Earths spin.Lavoisier (1743-1794), to create aequals exactly 1 dm 3 . On Oct. 16, 1967, the 13th CGPM unit of mass. It originally was calledRegarding time, in 1656, Chris- meeting decided that the SI unit of grave, meaning heavy. This name was changed to kilogram in 1795.On Sept. 28, 1889, the first meet-ing of the General Conference on Weights and Measures, also knownThis is the second in a series of articles about the metric system.as CGPM, determined that the in-ternational and national prototypesConversion of units of mass from the metric system of the kilogram would be madeto the U.S. customary system and vice versa.of the alloy of platinum with 10% iridium, accurate to within 0.0001%.1 kg = 2.204623 lbs. 1 lb. = 0.4535923 kgThe difference between the na-tional and international kilogram is1 kg = 35.27396 oz. 1 oz. = 0.02834952 kgwithin 1 mg (0.00004 oz.).The prototype of the kilogram is a right-circular bar, in which thetiaan Huygens (1629-1695), a Dutchtime is the second defined as fol-height equals the diameter, of 39.17physicist, mathematician, astrono- lows: The second is the duration of mm (1.54"). This bar is kept at themer and inventor, made the first9,192,631,770 periods of the radia-International Bureau of Weights andpendulum-driven clock. It had antion corresponding to the transition Measures and known as Le Grand K. error of less than one minute a day.between the two hyperfine levels of Themostaccuratemeasure- His later refinements reduced its er- the ground state of the cesium-133 ments revealed that the mass of therors to less than 10 seconds a day. atom.international prototype was a bitOn Oct. 14, 1954, the 10th CGPMOn Oct. 3, 1991, the 19th CGPM greater than the mass of 1 dm 3ofmeeting asked the Internationalmeeting noticed that the accu-water (about 0.028 g, or 0.001 oz.).Committee for Weights and Mea- racy of clock comparisons had im-Since it was decided to leave thesures to define the fundamental unitproved greatly through use of sat-international prototype unchanged,of time. ellite techniques. The conference the holding capacity of 1 L (33.8In 1955, Louis Essen (1908-1997),recommended that the coordinates fluid oz.), which is equal to the vol- an English physicist, built the firstadopted for antennas used in sin-ume of 1 kg (2.2 lbs.) of water at 4atomic clock based on transitiongle-way time comparison by satel-lite techniques correspond to their real position in the terrestrial refer-about the authorence system of the International Edmund Isakov, Ph.D., is a consultant, a CTE contributorEarth Rotation and Reference Sys-and the author of International System of Units (SI): Howtems Service. It is responsible for the World Measures Almost Everything, and the Peoplemaintaining global time, providing Who Made It Possible, which is available at https://books. data on Earths orientation and an-industrialpress.com. Contact him at 561-369-4063 ornouncing leap secondsthat is, [email protected]. one-second adjustments.CTE12MARCH 2020 MetricMatters.indd 12 2/14/20 10:37 AM'