The ampere (A) defines electric current (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 force per unit length of 2 x 10^{-7} N/m.

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

The coulomb can be rewritten as the ampere-second (A s), with s defined by the transition frequency of a caesium atom. 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 mass. 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 metre 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 (MSL) 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 metrology development.