Principles of Electric Circuits

Book Preface

In the 19th century, the principal weight and measurement units dealt with commerce.

As technology advanced, scientists and engineers saw the need for international standard measurement units. In 1875, at a conference called by the French, representatives from eighteen nations signed a treaty that established international standards. Today, all engineering and scientific work use an improved international system of units, Le Système International d’Unités, abbreviated SI*.

After completing this section, you should be able to

â—† Discuss the SI standard
â—† Specify the fundamental SI units
â—† Specify the supplementary units
â—† Explain what derived units are

Fundamental and Derived Units

The SI system is based on seven fundamental units (sometimes called base units) and two supplementary units. All measurements can be expressed as some combination of fundamental and supplementary units. Table 1 lists the fundamental units, and Table 2 lists the supplementary units.

The fundamental electrical unit, the ampere, is the unit for electrical current. Current is abbreviated with the letter I (for intensity) and uses the symbol A (for ampere). The ampere is unique in that it uses the fundamental unit of time (t) in its definition (second). All other electrical and magnetic units (such as voltage, power, and magnetic flux) use various combinations of fundamental units in their definitions and are called derived units. For example, the derived unit of voltage, which is the volt (V), is defined