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SI top research: Clocks, the kilogram and single electrons


The International System of Units (SI) is to be fundamentally reformed. In future, the SI will be determined by seven defining constants, among them Planck’s constant h and the elementary charge e. The base units the kilogram and the ampere, which are currently defined via an artifact (in the case of the kg) or via an idealized measurement instruction (in the case of the ampere), can then also be realized with the aid of such constants. (For the second and the meter, this reference to fundamental constants is already given today.) PTB is working intensively on corresponding high-precision realization procedures.

Such a fundamentally newly formulated system of units is at present being prepared in detail – both scientifically in the laboratories of the national metrology institutes and also under scientific and political aspects in the committees of the Meter Convention. In all likelihood, this reform of the system of units will be adopted at the next General Conference of the Meter Convention in 2018. In this package of redefinitions, the second will still maintain its current definition (based on a transition frequency in cesium). PTB is, however, already intensively working on the foundations of a future redefinition of the unit of time, the second, which will be based on optical transitions in atoms or ions.