Abstract :
It is one of the functions of the science and art of measurement to devise and apply methods of reliably carrying out the counting processes, and of defining and maintaining the magnitudes of the units counted. Some units, like the metre and the kilogramme, appear at first sight to be easy to maintain, as they can be stored on stable material standards. Derived units like velocity, pressure, or the electrical units are in general much more difficult to store, and in many cases can only be transitorily realised by the effect which they produce.
