New Definition of the SI Unit Kilogram – Spherical Interferometry as the Limiting Factor

Introduction

Metrology is the science of measurement and its application. Whenever one performs a measurement one compares the measurand to an appropriate unit. The number of these units has changed since the metre convention in 1875 to 1960 when the 11^th CGPM (Conférence Générale des Poids et Mesures) decided the actual SI (Système International d’Unités). Currently the SI is based on seven units – kilogram for mass, second for time, metre for length, ampere for electric current, Kelvin for thermodynamic temperature, candela for luminous intensity and – since 1971 – mol for amount of substance. Their definition also sometimes changes, due to demands on the unit or the advent of new methods for a possible better realisation. The metre, for instance, was incipiently defined as a part of a meridian of the earth. This definition was impractical to realise, so for the realisation the length between the ends of a platinum bar was used. To overcome the difficulties of the handling and measurement of this length standard, the definition was again revised to the distance between two marks on a new, x-shaped Pt-Ir rod. Although the measurement capabilities grew and even though interferometry was discovered early, the next redefinition took two ages. Eventually, in 1960 the metre was defined by a multiple of the wavelength of the radiation of ⁸⁶krypton. And since 1983, for the present finally, the metre is defined by a time-of-flight definition: one metre is the distance which light travels in 1/299792458 seconds where this special number is given by the fixed value of the speed of light.