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Why do we need to change the SI?

As science advances, ever more accurate measurements are both required and achievable. But this improving accuracy needs to be enabled by measurement standards and their definitions.

Continuously improving the definitions of the units ultimately makes it possible to have tighter tolerances and less waste. For example, gears will fit together better and therefore function more efficiently and manufacturing will be able to rely on the dimensions of parts to fit together.

The kilogram is the last SI base unit to be defined in terms of a human-made artefact, the International Prototype of the Kilogram (IPK). By definition the IPK always weighs one kilogram exactly. However, studies of closely similar copies tell us that the mass of the IPK is almost certainly changing … minutely. This implies a tiny but known change in the values of all masses. For mass, and for all units, we need to rule out this type of problem.

Constants of nature, such as the speed of light, are unchanging over time and space and we can give them exact values. Because of this, these constants provide the most stable and exact way to define all SI base units into the future.

Making this revision to the SI is a profound change in approach, that will underlie all measurements in science and more widely. But in everyday life it will appear that not much has changed. The new units will be the same size as previously but defined more precisely. The changes will ensure that the SI definitions remain robust for the future, ready for advancements in science and technology.

The new definitions impact four of the base units:

- The kilogram – will be defined in terms of the Planck constant (
*h*) - The ampere – will be defined in terms of the elementary charge (
*e*) - The kelvin – will be defined in terms of the Boltzmann constant (
*k*) - The mole – will be defined in terms of the Avogadro constant (
*N*_{A})

New definitions from May 2019

The revision of the SI comes into effect on 20 May 2019, World Metrology Day, when more than 80 countries will celebrate the impact measurement has on our daily lives.

**The kilogram is the SI unit of mass**

The kilogram is defined by taking the fixed numerical value of the Planck constant *h* to be 6.626 070 15 × 10^{-34} when expressed in the unit J s, which is equal to kg m^{2} s^{−1}, where the metre and the second are defined in terms of *c* and *∆ν*.

**The metre is the SI unit of length**

The metre is defined by taking the fixed numerical value of the speed of light in vacuum *c* to be 299 792 458 when expressed in the unit ms^{−1}, where the second is defined in terms of the caesium frequency ∆*ν*.

**The second is the SI unit of time**

The second is defined by taking the fixed numerical value of the caesium frequency ∆*ν*, the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s^{−1}.

**The ampere is the SI unit of electric current**

The ampere is defined by taking the fixed numerical value of the elementary charge *e* to be 1.602 176 634 × 10^{−19} when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆*ν*.

**The kelvin is the SI unit of thermodynamic temperature**

The kelvin is defined by taking the fixed numerical value of the Boltzmann constant *k* to be 1.380 649 × 10^{−23} when expressed in the unit J K^{−1}, which is equal to kg m^{2}s^{−2} K^{−1}, where the kilogram, metre and second are defined in terms of *h*, *c* and ∆*ν*.

**The mole is the SI unit of amount of substance**

One mole contains exactly 6.022 140 76 × 10^{23} elementary entities. This number is the fixed numerical value of the Avogadro constant, *N _{A}*, when expressed in the unit mol

**The candela is the SI unit of luminous intensity**

The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 10^{12} Hz, *K _{cd}*, to be 683 when expressed in the unit lm W

© National Physical Laboratory 2018

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