Appendix A
SI Units of Measurement and Their Realisation at NPL
| Quantity | Unit (Symbol) | Definition | Realisation |
| Time | second (s) | The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. | The second is realised by primary caesium frequency standards to about 2 parts in 1015. The majority are traditional caesium-beam designs, but the latest use lasers to control and detect the atoms. |
| Length | metre (m) | The metre is the length of the path travelled by light in a vacuum during a time interval of 1/299 792 458 of a second. | At NPL, the metre is currently realised through the wavelength of the 633 nm radiation from an iodine-stabilised ... |
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