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Redefinition of the International System of Units

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Redefinition of the International System of Units Redefinition of the International System of Units Martin Hudlička Czech metrology institute [email protected] CTU FEE K13117 Departmental seminar, 12th February 2019, Prague Overview • SI history • Measurement traceability • The need for change of SI units • Preparations for change – the kilogram – the mole – the ampere – the kelvin • Consequences for users • Further reading CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • various units used for thousands of years, some of systems stable over time • mostly based on nature or human body measures (which were thought to be constant) – Babylonian system (example – units of length) source: Wikipedia CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history – Egyptian system measurement of length, volume, mass, time bronze capacity weight measure standard source: Wikipedia CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • many other official measurement systems for large societies (Olympic system of Greece, Roman system, ...), some of the systems adopted into later systems • English system – measurements used in England up to 1826, huge number of units for various areas – liquid measures (dram, teaspoon, tablespoon, pint, gallon, ...) – length (digit, finger, inch, nail, palm, hand, ...) – weight (grain, pound, nail, stone, ounce, ...) • Imperial system (post-1824) used in the British Empire and countries in the British sphere of influence credit: D. Pisano, Flickr CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • United States customary units – based on English units, however significant differences exist – example: liquid volume CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • major problem – multiple units, conversion between units usually not decimal credit: METAS elbow foot span CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • major problem – multiple units, conversion between units usually not decimal CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • the metric system developed from 1791 onwards by a committee of the French Academy of Sciences (use of decimal system and prefixes to express multiples), many countries joined, metre and kilogram prototypes owned by French government • 1875 – Metre convention, signed by 17 countries (only the metre and the kilogram), a set of 30 prototypes of the metre and 40 prototypes of the kilogram made from 90% Pt and 10% Ir alloy manufactured in 1889 source: Wikipedia CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • one kilogram = the same mass as one litre of water (atmospheric pressure, temperature 4 °C) • one metre = distance between two lines marked on the bar (originally ~1670 derived from a pendulum, from ~1790 defined as 1/10 000 000 of the distance between the North Pole and the Equator going through Paris) • metre definition not ideal (meridian not uniform across the world) source: PTB museum CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • the General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), established in 1875, brought together many international organisations to establish the definitions and standards of a new system • ~1900 - three different systems of units of measure existed for electrical measurements – CGS (centimeter, gram, second) system for electrostatic units, known as Gaussian or ESU – CGS-based system for electromechanical units (EMU) – system based on units defined by the Metre Convention • 1921 – attempt to add other physical units to the Metre convention too (especially electrical quantities) • ~1945 - number of different systems of measurement used throughout the world (metric system variations, U.S. customary system, Imperial system) – change needed CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • 1960 - the 11th CGPM synthesised a set of 16 resolutions, the system was named the International System of Units (Le Système International d'Unités, SI) • the metre, kilogram, second, ampere, degree Kelvin, and candela • base units derived from invariant constants of nature (e.g. speed of light in vacuum), triple point of water and one artefact (kilogram) • derived units - defined in terms of base units or other derived units (e.g. 1 Pa = kg·m-1·s-2, 1 Ω = kg⋅m2⋅s−3⋅A−2 etc.) • 1971 – mole added to the six units CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • the 1960 system of units valid until 2018, the “new” system effective from May 2019 credit: METAS CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the kilogram (mass) The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. • 1 international prototype (Paris) • 6 sister copies (Paris) • 10 working copies (Saint-Cloud, France) • national prototypes (about 69 pieces in various countries) • comparison to the international prototype every ~40 years CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the metre (length) The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second. • can be realized in three ways – measurement of time which the light needs to travel the measured length – using of wavelength of radiation, frequency of which is measured – using a wavelength of a primary standard – laser stabilized to one of transitions, whose values and uncertainties are given by BIPM CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the metre (length) • Czech Republic – quantum length standard based on helium- neon laser ( = 633 nm, = 612 nm and = 543.5 nm) stabilized to hyperfine iodine transitions credit: CMI CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the metre (length) • frequency of laser compared to the caesium standard by means of frequency comb generator and interferometry - beat signal measurable in Hz, kHz or MHz range (depending on laser wavelength) CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the second (time) The second is the duration of 9192631770 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. • Czech Republic – Institute of Photonics and Electronics , Czech Academy of Science • use of two commercial 5071A caesium clocks (relative stability ~5·10-13 to the ideal realization) credit: CAS CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the second (time) - atoms emit and absorb electromagnetic waves - transition between two energy states -> photon is emitted or absorbed with frequency f0 = (E2–E1)/h - atoms prepared in one state, population of the other state registered - f0 ~ 9.2 GHz for caesium - definition introduced in the 1960s, proved to be stable over time credit: PTB CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the ampere (electric current) The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 newton per metre of length. credit: PTB CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the ampere (electric current) • direct realization – current balance (Ampère, ~1900) • Lorentz force acts between moving (blue) and non-moving (red) coil, compensated by gravitational force • relative uncertainty ~5 ppm, dependent on accuracy of kg NIST, 1912 • non-uniform current distribution in wires and magnetic field distribution in space CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the ampere (electric current) • indirect realization – Ohm’s law I = U/R, Josephson effect and quantum Hall effect enable very stable realization of units [volt] and [Ω] • not part of SI, thus not recognized as a realization -> see later in the “quantum SI” CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the kelvin (thermodynamic temperature) The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. This definition refers to water having the isotopic composition defined exactly by the following amount of substance ratios: 0.00015576 mole of 2H per mole of 1H, 0.0003799 mole of 17O per mole of 16O, and 0.0020052 mole of 18O per mole of 16O. CTU FEE K13117 Departmental seminar, 12th February 2019, Prague SI history • definition of SI units (before 2019): the kelvin (thermodynamic temperature) • direct realization of one kelvin (t [°C] = T - 273.15) – triple point of water • Czech Republic – temperature scale -196 °C to +1048.6 °C (triple points and melting points of various elements H20, N, Hg, Cu, ...) credit: CMI CTU FEE K13117 Departmental seminar, 12th
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