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On the hunt for a new ampere

Counting charge one electron at a time

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While there's been lots of attention paid to the search for a new kilogram, another of the SI system's fundamental units of measurement is under examination: the ampere.

Along with the kilogram, metre, second, Kelvin, mole and candela, the ampere is one of the fundamental yardsticks used to measure the world around us. And, like the kilo, the ampere is suffering from the incredible accuracy of modern scientific instruments: its value differs ever so slightly under different measurement conditions.

In a much older world, the amp was measured in terms of vast numbers of electrons passing through a wire. One coulomb per second equals one amp. That, however, equates to an awful lot of electrons passing a point, and isn't particularly accurate in today's world, where you can create stop-motion movies with single atoms.

To get a more stable ampere, the Conférence Générale des Poids et Mesures proposes that the unit be defined in terms of electron charge. For example, a single electron pump, or SEP to its friends, can emit single electrons at a controllable rate by dropping them onto quantum dots.

However, metallic SEPs don't pump electrons fast enough to provide a usable ampere, so scientists from the National Physical Laboratory and the University of Cambridge have constructed one out of graphene, and are putting it forward as their suggested “SI ammeter”.

As their paper in Nature (abstract here) explains, the UK scientists believe their graphene-based SEP is accurate at the gigahertz range, which is sufficient to provide a new physical basis for the ampere.

However, in a demonstration of the Fundamental Interconnectedness of All Things*, regardless of what technique is eventually used to measure the ampere, it'll have to wait until the new definition of the kilogram is settled – because that will also nail down the SI value for electronic charge, on which the ampere will be based. ®

*Thanks to Douglas Adams ®

Update: a reader has sent The Register the following comment, which is worth reproducing:

"The ampere is not (and never has been) defined as the number of electrons passing a given point (that's a definition of the coulomb, a derived unit). All base units must be defined in terms of mass, length and time and here's the ampere: The current flowing in two rectilinear conductors of negligible cross-section 1 metre apart in vacuuo that produces a force of 2*10^-7 Newtons between them. There are issues with this due to measurement accuracy and the desire for a new definition has been desirable for some years."

Our thanks. ®

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