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The new International System of Units (SI)


The International System of Units (SI) is about to undergo a fundamental revision. In the future, the values of certain selected fundamental constants will be set; the units will then be derived from these values. For two units, this has already taken place: the meter is defined via the speed of light and the second via a special frequency in the cesium atom. Neither of these two units will go through any essential changes. Quite the opposite, though, is true of the kilogram, the mole, the ampere and the kelvin: their changes will be fundamental in nature. The fundamental constants that are relevant for these units are Planck’s constant, the Avogadro constant, elementary charge and the Boltzmann constant.

The metrological background factors and implications from this revision of the SI form one element. Providing information on this reform to the broadest possible spectrum of experts, to schools and universities, and to the general public forms another element. It is this dissemination of information that PTB takes very seriously, developing a range of publications for its different target groups and several event formats for this complex subject. These publications can be downloaded from Opens internal link in current windowhttp://www.ptb. de/cms/en/research-development/research-on-thenew- si.html or ordered from the Press and Information Office. This website also contains all available PTB info sheets in all matters related to the new SI.