New Ampere won't affect your measurements

-April 18, 2014

You may have heard talk about the Ampere being redefined. Rest assured that unless you work at the highest levels of metrology, the change in how the Ampere is realized won't affect you.

The present definition of the Ampere is:

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.

That's nice, but rather theoretical. More practical methods rely on Ohm's Law using a voltage produced by a Josephson Junction Array. Other methods include single-electron tunneling, and by applying a voltage ramp to a capacitor. These methods are described in CCEM/09-05.

Discussion of redefining the Ampere will take place in November with the possibility of a new definition based on single-electron tunneling. Even if metrologists and physicists agree on a definition, it will take several years before the definition of the Ampere will be implemented. The concept of measuring current using single electrons was reported as early as 2007, with a new method reported in December 2013.

Will a change in the definition of the Ampere affect everyday measurement? According to Jeff Gust, Chief Corporate Metrologist at Fluke Calibration, any change in the definition will affect only measurements at the top of the calibration chain. That is measurements on current with uncertainties of 10-8 (0.01 ppm) or lower. Gust wrote a paper in 2011 where he discussed the impact of proposed changes to SI units.

Changing the definition of the Ampere will cause a ripple effect because the Ampere is used to derive numerous other measurement units, including voltage, capacitance, inductance, magnetic flux, and flux density. Take al look at all the units defined using the Ampere in the NIST Guide to the SI. But, the changes will be minute with regard to everyday measurements. So, go on and keep using your DMMs and the calibrators used to calibrate them.

Everyday current measurement, such as those made by clamp meters,
don't have uncertainty small enough to be affected by a new Ampere definition.

Also See
Follow the Chain to NIST-Traceable Calibrations
AC metrology at the top of the chain
Track your unbroken chain to NIST
NIST closed, all measurements invalid

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