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    • Rights: Measurement Standards Laboratory of New Zealand
    Published 15 August 2019 Referencing Hub media
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    The ampere1 (A) defines electric current2 (iahiko).

    The ampere was once defined as the current flowing in two very long parallel wires that are 1 m apart and which gives rise to a magnetic force3 per unit length of 2 x 10-7 N/m.

    Like many of the other SI4 units, the ampere’s definition was updated in May 2019. Now it is linked to the elementary charge (e), which is the electric charge5 carried by a single electron6 – e has a fixed value of 1.602 176 634 x 10–19 coulomb7.

    The coulomb can be rewritten as the ampere-second (A s), with s defined by the transition frequency8 of a caesium atom9. This link allows one ampere to be defined solely in terms of fundamental constants – it is the electric current corresponding to the flow of 1/(1.602 176 634 x 10–19) elementary charges per second.

    Discussion point: The electron charge constant (e) is extraordinarily small. On the positive side, the ampere is defined by a constant. What might be some difficulties in using such a small constant?

    Transcript

    FARZANA MASOULEH

    Next unit of [measurement] is the ampere. Befitting the name, the logical constant to define the ampere is the electron charge. And the electrical community has been ready to do this redefinition for a long while now, but they have not been able to do so and that has a reason. The reason is that the electron charge – the reason relies on the electron charge definition because it is indirectly related to force through mass10. So if we don’t know mass based on a physical constant, then we will not be able to measure the ampere accurately.

    This equation, this slide shows the old definition for ampere, which is, which one ampere is the current in two parallel wires which are infinitely long and stay 1 metre11 apart while a force of 2 x 10-7 N m is applied between them. But – and that’s the electron charge constant which we are not sure, that’s why we have the question marks there – but after 20th May [2019], we will make the electron charge constant to the most accurate result that we’ve got so far and then we will not be sure what is the current in that wire, although it is very close to 1 ampere still, but it won’t be 1 absolute ampere any more.

    Acknowledgements

    This video clip is from a recording of a presentation by the Measurement Standards Laboratory of New Zealand (MSL12) in celebration of the redefinition of the International System of Units (SI), which happened on 20 May 2019. The presentation by Peter Saunders and Farzana Masouleh of MSL was filmed at Unleash Space, Faculty of Engineering, Auckland University.

    Filming and editing by Jonathon Potton of Chillbox Creative. MSL produced these videos to share the story of metrology13 development.

    1. ampere: The basic unit of electrical current in the International System of Units (SI).
    2. current: The flow of electric charge through a conductor.
    3. force: A push or a pull that causes an object to change its shape, direction and/or motion.
    4. SI: A measurement system used worldwide in the scientific community. SI stands for Système International d’Unités.
    5. electric charge: An excess of electrons on an object gives it a negative charge whereas a deficiency of electrons gives it a positive charge. Protons carry a positive charge and electrons carry a negative charge. Ions carry a positive or negative charge.
    6. electron: A light subatomic particle with negative charge, found in the space surrounding an atomic nucleus.
    7. coulomb: Unit of electric charge derived from the ampere. It is defined as the charge transported by a steady current of 1 ampere in 1 second.
    8. frequency: 1. How often something occurs within a specified time. 2. The number of waves per second that pass a given point or the number of waves produced per second by a source.
    9. atom: The smallest possible unit of matter that still maintains an element’s identity during chemical reactions. Atoms contain one or more protons and neutrons (except hydrogen (H), which normally contains no neutrons) in a nucleus around which one or more electrons move.
    10. mass: The amount of matter an object has, measured in kilograms.
    11. metre: The base unit of length in the International System of Units (SI).
    12. MSL: The acronym for the Measurement Standards Laboratory.
    13. metrology: The science of weights and measures.

    Appears in

    INTERACTIVE

    The International System of Units (the SI)

    Information about the International System of Units – the SI – including their definitions using fundamental constants.

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      ampere

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      2. The basic unit of electrical current in the International System of Units (SI).

      SI

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      3. A measurement system used worldwide in the scientific community. SI stands for Système International d’Unités.

      coulomb

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      4. Unit of electric charge derived from the ampere. It is defined as the charge transported by a steady current of 1 ampere in 1 second.

      mass

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      5. The amount of matter an object has, measured in kilograms.

      metrology

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      6. The science of weights and measures.

      current

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      7. The flow of electric charge through a conductor.

      electric charge

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      8. An excess of electrons on an object gives it a negative charge whereas a deficiency of electrons gives it a positive charge. Protons carry a positive charge and electrons carry a negative charge. Ions carry a positive or negative charge.

      frequency

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      9. 1. How often something occurs within a specified time.

      2. The number of waves per second that pass a given point or the number of waves produced per second by a source.

      metre

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      10. The base unit of length in the International System of Units (SI).

      force

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      11. A push or a pull that causes an object to change its shape, direction and/or motion.

      electron

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      12. A light subatomic particle with negative charge, found in the space surrounding an atomic nucleus.

      atom

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      13. The smallest possible unit of matter that still maintains an element’s identity during chemical reactions. Atoms contain one or more protons and neutrons (except hydrogen (H), which normally contains no neutrons) in a nucleus around which one or more electrons move.

      MSL

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      14. The acronym for the Measurement Standards Laboratory.

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