3  SI

The SI (Système International d’Unités) is the modern form of the metric system and the most widely used system of measurement for science and engineering. It defines seven base units:

• meter for length,
• kilogram for mass,
• second for time,
• ampere for electric current,
• kelvin for temperature,
• mole for amount of substance,
• candela for luminous intensity.

All other units are derived from these basic seven units.

The SI provides a standardized way to express and compare physical quantities across disciplines and countries.

3.1 other systems

Before the French Revolution, a person traveling across towns and countries would encounter a myriad of local system of units. Most often, the names for the units used in neighboring towns were exactly the same, but the exact quantities were not. So a pound of cheese here might be 20% higher than a pound from elsewhere.

While most of the world uses today the SI units, the United States and a few other countries still use older unit systems.