Mankind has a fascination with measurement. Down the centuries we have produced a plethora of incompatible and duplicatory systems for measuring everything from the width of an Egyptian pyramid to the concentration of radioactivity near a nuclear reactor and the value of the fine structure constant. With the introduction first of the metric system and of its successor the Système International d'Unités (SI), the scientific community has established a standard method of measurement based on only seven core units. The Encyclopaedia of Scientific Units, Weights and Measures converts the huge variety of units from all over the world in every period of recorded history into units