1. Connect, but connect to what?

To understand the importance of connecting measuring instruments, we need to remember that the system of measurement units is not "natural". Although we often feel we know the metre, the second or the kilogram, most of us would be hard pressed to define them exactly. These units are part of our daily lives, they seem obvious to us, but we, as citizens, have lost their origins.

It was the French Revolution that instituted the international system of units. Today, this system evolves under the authority of the General Conference on Weights and Measures (CGPM). This organization defines and ensures the international uniformity of measurement units. The chain of metrological traceability begins with the unit definitions laid down by the CGPM, and runs down through a succession of connections (usually in the form of calibrations) to the end user.