The basis of all measurements also is a basis of PTB's work: (almost) everything revolves about the seven SI base units. PTB maintains the German copy of the "original kilogram", the international prototype kilogram. The time is "made" in the PTB headquarters in Braunschweig, the temperature by one of the PTB divisions domiciled in Berlin. Under the Units Act, PTB is bound to realize and disseminate the legal units and to maintain the respective embodiments (prototypes and standards). This task has an impact on the four fields of work PTB: Upon approval (coming under 
legal metrology) or upon calibration (part of the services for industry) measuring instruments must, for example, be linked up with the respective national standard. The comparison of its standards with those of other countries is among the tasks which PTB performs on the international level. Finally, it is the task of fundamental research to constantly look for better methods for the realization and dissemination of the units.
A special challenge is the activities regarding the traceability of SI units to fundamental constants. This traceability has already been successfully achieved for the second, the meter, the volt and the ohm, as - at least according to today's understanding of physics – then the assumption can be made that realization of the unit is independent of time and place. By the decision of the Comité International des Poids et Mesures (CIPM) to introduce preparatory measures for the redefinition of the units ampere, kelvin, kilogram and mole on the basis of fundamental constants, this work has gained additional importance and topicality. Central starting point is the precise determination of the values of the fundamental constants which serve as a basis. Here, too, PTB makes important contributions with very sophisticated experiments Avogadro constant, Boltzmann constante.
From the following links you can put together your personal tour all about the SI base uni
- The International System of Units (SI)
- The seven SI base units:metre , kilogram, second, ampere, kelvin, mole, candela
- The history of the SI
- The metric system
- A basis for the units
Additional information:
Very interessting information (e.g. the brochure "The International System of Units as PDF or HTML) can also be found on the 
SI-pages of BIPM
The Swiss Metrology Institute METAS also offers very informative pages all about the seven base units: http://www.metas.ch/metasweb/Themen/Masseinheiten
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