The Electrical Connection François Piquemal Tells the Story of the Ampere, Which Bridges Mechanical and Electromagnetic Units

measure for measure The electrical connection François Piquemal tells the story of the ampere, which bridges mechanical and electromagnetic units.

n everyday electrical life, watt and volt In reality, international standards remained crucial role in the planned redefinition of the probably mean more than ampere: when at home and metrological institutes used ampere (and the kilogram)5,6. Ireplacing a lamp, you need the right secondary, transportable standards. History seems to repeat itself. Like wattage, when changing a battery, voltage Dissatisfaction about expressing the past schism between the practical is what you check. A quick explanation of electrical quantities in a limited 3D system international electrical units and the the ‘amp’ is that a current of 1 A generates of (mechanical) units soon became a CGS system, the conventional (quantum- a power of 1 W in a conducting element to critical issue. In 1901, Giovanni Giorgi had standard) ohm and volt — in use since

which a voltage of 1 V is applied. demonstrated the possibility of designing one 1990 and based on defined values ofK J and This definition is equivalent to the one coherent 4D system by linking the MKS units RK (ref. 5) — are not embedded in the SI. approved during the first International with the practical electrical system via a Fortunately, progress in metrology is being Exposition of Electricity held in Paris in single base electrical unit from which all made, and a revision of the SI, solving this 1881: the ampere is the current produced by others could be derived. After some debates, and other issues, is planned for 2018 (ref. 6). one volt in one ohm. The name ampere — the ampere was preferred to the ohm as the In the new SI, the ampere will be defined as after André-Marie Ampère, the founder of connecting unit, leading to the well-known the electric current corresponding to the flow electrodynamics (pictured) — was chosen to MKSA system adopted from 1948 and of 1/(1.602176xyz × 10−19) elementary charges avoid confusion: at the time, a unit named superseded by the Système International per second, with the elementary charge weber (after Wilhelm Weber) quantifying d’Unités (SI) in 1960. The definition of the fixed at 1.602176xyz × 10−19 C. The three last electrical current was in use in both the ampere as the unit of electric current significant digits,xyz , are not settled yet. United Kingdom and Germany, but was then given its present The new definition of the ampere is ‘future- differing by a factor of ten1! form4: “that constant current proof’ as it does not imply a way for realizing O T Formulating electromagnetic O which, if maintained in two the ampere — unlike the 1948 definition. A H P measurements in terms of straight parallel conductors straightforward way is via quantum realizations K C mechanical quantities (length, O of infinite length, of of the volt and the ohm. A still very challenging T S

mass and time) was pioneered Y negligible circular cross- method is based on the very definition of

by Carl Friedrich Gauss, L section, and placed current itself: measuring the quantized flow

who in 1832 expressed his 1 metre apart in vacuum, of charges in a certain time interval in a

T L

result for the intensity of would produce between nanodevice in a controlled way. Interestingly,

the Earth’s magnetic force R these conductors a force the first method relies on collective phenomena P

−7

using the millimetre, the L equal to 2 × 10 newton accounted for by the wave nature of electrons, A

2 R

milligram and the second — O per metre of length.” whereas the other approach involves a device

P ❐ the starting point for the Because of the low based on the electron’s corpuscular nature!

centimetre–gram–second (CGS) © achievable accuracy (parts in system, later replaced by the metre– 106 at best) when implementing FRANÇOIS PIQUEMAL is at the Fundamental kilogram–second (MKS) system. such a thought experiment, Electrical Metrology Department, By the early twentieth century, the need realizations of the ampere are derived Laboratoire National de Métrologie et had arisen to express the electrical units in in practice from the ohm and the volt or, for d’Essais, 29 avenue Roger Hennequin, terms of physical standards, and a new system currents less than 100 pA, from the farad, the 78197 Trappes CEDEX, France. of international units for practical use was volt and the second. e-mail: [email protected] introduced. The international ampere was The discovery of two condensed-matter defined as the current that deposits a silver quantum phenomena, the Josephson effect References mass of 0.001118 grams per second (ref. 3) in 1962 and the quantum Hall effect in 1. Congrès International des Électriciens Paris 1881, Compte Rendu des Travaux [in French] 44 (Masson, 1882). on the cathode of a silver nitrate electrolyser. 1980, marked the beginning of a new era 2. Gauss, C. F. Intensitas vis Magneticae Terrestris ad Mensuram It was envisaged that such standards would in metrology. These effects enable accurate Absolutam Revocata [in Latin] 2041–2058 (Göttingische Gelehrte travel between countries and be compared measurements of the Josephson constant, Anzeigen, 1832). to each other. For the international ohm, K = 2e/h, and the von Klitzing constant, 3. Proceedings of the International Electrical Congress Held in the City J of Chicago: August 21st to 25th, 1893 17 (American Institute of 2 this was a delicate affair: the standard was RK = h/e , and hence determinations of Electrical Engineers, 1894). based on a column of mercury (with a cross the elementary charge, e, and the Planck 4. SI Brochure: The International System of Units (SI) 8th edn section of 1 mm2 and a length of 106.3 cm) — constant, h. Highly reproducible present-day (BIPM, 2006); http://www.bipm.org/en/publications/si-brochure/ 5. Piquemal, F. in Handbook of Metrology (eds Glaser, M. & unimaginable today given the international quantum standards for the volt and the ohm Kochsiek, M.) Ch. 9, 267–314 (Wiley, 2010). regulations concerning mercury’s toxicity! are based on these phenomena — and play a 6. Fischer, J. & Ullrich, J. Nature Phys. 12, 4–7 (2016).

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