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Bartlett's Guide to SI Units

Contents

1. Introduction
2. Base and Supplementary SI Units
3. Derived Units
4. SI Prefixes
5. Other units used with the SI
6. A guide to correct usage of SI Units
7. A brief History of the Metric System (SI)

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1. Introduction

The Système International d'Unités (SI), the modern form of the metric system, is the most widely used system of units and measures around the world. In spite of this there is widespread misuse of the system with incorrect names and symbols used as a matter a course - even by well educated and trained people who should know better. For example how often is mHz, Mhz or mhz written when referring to CPU clock rates? The correct form is actually MHz. Note that the capitalisation does matter.

This brief reference guide attempts to give accurate but concise information to clarify some of the mysteries of SI units and their conventions. It is not meant to be an exhaustive treatise on the subject, for this please refer to some of the definitive official publications eg National Institute of Standards & Technology (USA).

The SI comprises units and SI prefixes. The prefixes are used to form decimal multiples or sub-multiples of the units. The units are classified into Base units, Supplementary units and Derived units. The following sections described the different elements.

2. Base and Supplementary SI units

3. The Derived units of the metric system (SI)

4. SI Prefixes

5. Units that may be used with the SI

6. Correct usage of the SI symbols

The following points emphasise some of the important aspects about the use of SI units and their symbols, and also indicates some of the common errors that are made. The SI unitary system differs from some of the older systems in that it has definite rules governing the way the units and symbols are used.

• The name of a unit always starts with a lower case letter, unless it is the first word in a sentence, where normal grammatical rules apply. This is true even if the symbol uses upper case. For example pascal and Pa are correct.
• The case of the letters in a symbol is important. For example m and M mean distinctly different things. MHz is megahertz (millions of cycles per second) while mHz is millihertz (thousandths of cycles per second). Computer clocks are typically MHz. A mHz computer is a very slow one. :))
• The unit of measure is the metre, not meter. The latter is a device used for measurement. (Unless you live in the USA - in which case you will just have to live with the ambiguity.)

7. A brief History of the metric system (SI)

As early as 1584 Simon Stevenius had already proposed a decimal system of units and money in his book De Thiende. However, it was not until the French Revolution that the climate was conducive to creating a completely new system of units. In 1790 the French Academy of Science was commissioned by the National Assembly to design a new system of units for use throughout the world. They decided that this system should have the following attributes:

1. the system should consist of measuring units based on invariable quantities in nature,
2. all units other than the base units should be derived from these base units,
3. multiples and sub-multiples of the units should be decimal.

These principles still underpin the modern metric system (SI).

France created world-wide interest with this development and it resulted in 15 countries subscribing to the Metre convention in 1875. Through this the Bureau International des Poids et Mesures (BIPM) came into being. The BIPM now functions under the guidance of the Conférence Générale des Poids et Mesures (CGPM) which has delegates from all the countries that have subscribed to the convention.

Over the years the metric system has evolved, and in 1960 at the 11th CGPM the system was officially named the Système International d'Unités, abbreviated as SI . The SI is the logical evolution of the metric system and replaces all previous metric systems. It is a dynamic system which is continually being revised to keep pace with developments in science and technology.