# 5.1.4 Alternate Units of Measurement—the Système Internationale

There are many different units used throughout the world, but scientists and technologists of all nations have agreed to use a standard system of units called the Système Internationale (International System), known as SI. One major advantage to the SI is that everything you could ever want to measure can be measured using a few basic units, or combinations of them. [ Did you know that the U.S. actually began to convert to the metric system in the 1970s? As a result, there are quite a few cartoons out there that discuss and poke fun at these systems. ]

Although the U.S. has not adopted the SI, metric units are still in use. For example, perscription drugs will have  the dosage  stated in milligrams (mg), an SI measure of weight.

It’s really important not to confuse metric and Imperial units. In 1999, a NASA Mars Orbiter space craft was destroyed on arrival in the Martian atmosphere at a loss of \$125m. An inquiry established that the flight system software on board the Orbiter was written to calculate thruster performance in metric Newtons (N), but mission control on Earth was inputting course corrections using the Imperial measure, pound-force (lbf). An expensive mistake …

To show you how the SI system works, we shall look at the units of length. The SI base unit for length is the meter, symbol “m.” It is useful to know a rough idea of the size of a typical measurement in any given unit, so, for instance, the height of a typical domestic door is about 2 meters.

The meter was originally intended to be one ten-millionth of the distance from the Earth’s equator [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] to the North Pole (at sea level), but since 1983 it is defined as the length of the path travelled by light in vacuum in of a second.

5.1.3 Volume Measurements

5.1.5 The Metric System