SI Unit Prefixes
Now that we know about the SI system and what it provides to the scientist and engineer, we can examine some aspects of actual measurement. The SI system utilizes a standard system of prefixes to the basic units that allow them to be more relevant to and descriptive of relative magnitude.
For example, when reading about chemical kinetics, one often encounters the the terms "ms" or "ns," meaning "millisecond" and "nanosecond," respectively. Once you've become accustomed to the practice of using the prefixes, you will immediately know that the millisecond is 1/1000 of one second and is 1 million times larger than a nanosecond, which is 1/1000000000 of one second, or 10-9 seconds.
Recall the basic SI units, represented in the following illustration:
The prefixes, tabulated in the below illustration, allow interpretation of the unit as either a multiple or a fraction of the base unit.
Notice that there are 20 accepted prefixes. A prefix may be added to a unit to produce a multiple of the original unit. All multiples are integer powers of ten, and beyond a hundred(th) all are integer powers of a thousand. For example, kilo- denotes a multiple of a thousand, and milli- denotes a multiple of a thousandth; hence there are one thousand millimeters to the meter and one thousand meters to the kilometer. The prefixes are never combined. Thus a millionth of a metre is a micrometer, not a millimillimeter, and a millionth of a kilogram is a milligram, not a microkilogram. In older usage, a micron (a measurement often encountered in physics and engineering) is the same as a micrometer, 10-6 meters. Another older form of usage, the millimicron, is one thousandth of a micrometer, or 1 thousandth of 10-6 meters, or 10-9 meter, now called a nanometer. While these older terms are not in common usage, they are often encountered in older publications, and knowing their modern equivalents is an advantage.