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 are due to come into force with effect from 20 May 2019.

The kilogram, ampere, kelvin, and mole will then be defined 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 are already defined in terms of physical constants, but their present definitions are likely to see minor revisions in their wording.

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

The Ampere

Since 1948, the ampere has 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, with effect from 29 May 2019, the revised definition will be 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)

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