News about the ampere

©2019 L A Waygood

Redefinition of the ampere

On 16 November 2018, the 26th General Conference on Weights and Measures  (CGPM) voted unanimously in favour of revised definitions of the seven SI Base Units, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year. The new definitions came into force from 20 May 2019.

However, it should be understood that the actual changes to SI are far more radical than simply changes to the base units, because the foundation of the ‘new’ SI system has shifted away from the base units to explicitly-designated physical constants.

So when the new system took effect on 20th May 2019, every SI unit —not only the base units but also derived units such as the volt, the newton, the weber, and more— will be derived by some combination of seven invariants of nature.

So the kilogram, second, meter, ampere, mole, kelvin, and candela will essentially lose their special status as ‘base units’. Though those units will continue to be referred to as ‘base units’, the SI ‘derived units’ will also be directly tied to the foundational constants —in other words, derived units will no longer necessarily be defined in terms of the old base units!

For decades SI units were defined via seven base units. From May, 2019, all units will be defined via one or more of seven constants, which will be assigned exact values.

The kilogram, ampere, kelvin, and mole were redefined by setting exact numerical values for the Planck constant (h), the elementary (electric) charge (e), the Boltzmann constant (k), and the Avogadro constant (NA), respectively.

The metre and candela were already defined in terms of physical constants so they didn’t require redefining, but there have been minor revisions in their wording.

The new definitions were intended to improve the present SI definitions without changing the size of any unit, thus ensuring continuity with existing measurements

The ampere (A)

Since 1948, the ampere had been defined in terms of the resulting force between two, parallel, current-carrying conductors, due to the attraction or repulsion of their magnetic fields, as follows:

The ampere is defined as ‘the constant current that, if maintained in two straight parallel conductors of infinite length and negligible cross-sectional area and placed one metre apart in a vacuum, would produce between them a force equal to 2 × 10-7 newtons per unit length’.

But,  from 29 May 2019, the ampere is defined in terms of the rate of flow of individual elementary charges (electrons).

The revised definition is as follows:

The ampere, symbol A, is defined by taking the fixed numerical value of the elementary charge, e,  to be 1.602176634×10−19 when expressed in the unit coulomb (C), which is equal to an ampere second (A⋅s), where the second is defined in terms of ΔνCs.

(The symbol, ΔνCs   ,represents the fixed numerical value of the caesium frequency, the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit hertz, which is equal to s−1)

By defining the ampere solely in terms of the elementary charge, e, direct measurements of the ampere now becames a matter of counting the transit or passage of individual electrons in a device over a given period of time.

So, the effect of this definition is that one ampere is the electric current corresponding to a flow of 1/(1.602 176 634 x 10–19) elementary charges per second.

The elementary charge, e is unimaginably small ―about a tenth of a billionth of a billionth of the amount of charge in a current of one ampere that moves past a given point in one second. Measuring individual electrons past a point is a major challenge for scientists is to produce a current of individual electrons that can be routinely measured and used as a standard.

So, although the new definition has finally put the ampere on a more rational footing, it poses new and formidable challenges for measurement science. Unfortunately, too, unlike the previous definition is has made it more difficult for the student to understand!

The 9th edition of the SI Brochure (2019), which defines and presents the Système International d’Unités, the SI (known in English as the International System of Units) is downloadable from the Bureau International des Poids et Mesures (BIPM) website.


Please note that those who have purchased my books, An Introduction to Electrical Science and Electrical Science for Technicians should amend the definition accordingly.