The magnetic permeability puts on weight !

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Key facts and impacts on the new quantum definitions for SI units.

This November 13-16th, the General Conference on Weights and Measures (CGPM) held in Versailles, officially approved the definition of all seven units in the SI system based on physical constants. The new definition comes into force on 20th May 2019.

This decision concludes a revolution started more than half a century ago, for the sake of reaching a more precise and reliable metrology system. Indeed, precise and universal measurements are unreliable and awkward if defined on standard quantities such as the standard meter and the standard kilogram, kept physically at one place in the world. Quantum mechanics now plays the key role in the definition of units, which can be thus implemented with high precision and at any place.

The time was first redefined in 1967, based on the transition frequency between two hyperfine levels of the Cesium 133 atom, considered in the ground state at 0K. The meter was redefined in 1983, based on the distance that light travels during a given fraction of a second, fixed as 1/299 792 458.

The new changes deeply affect us magneticians. The kilogram is now defined based on Planck’s constant, which should equal 6.626 070 15·10-34 when expressed in J·s, which is the same as kg·m2·s-1. The kilogram is therefore now linked with the underlying quantification of angular momentum, which is a key aspect of magnetism as it determines the Bohr magneton. Besides, the Ampere is now defined from the charge of the electron e, being set to 1.602 176 634·10–19 C or A·s, while so far it was defined as the force acting between linear currents.

Passed the philosophical move, a practical consequence for our community is on the permeability of vacuum µ0. Still µ0 ε0 c2=1, however, it is not anymore strictly equal to 4π·10-7, however it needs to be determined experimentally from µ0=2αh/(c e2). The latest measurements yield:


= 4π [1 + 2.0(2.3) ·10–10] · 10–7 N·A–2

= 12.566 370 6169(29) · 10–7 N·A–2

Olivier Fruchart.

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