Wednesday, November 18, 2009

SI prefixes

The 11th CGPM (1960, Resolution 12) adopted a series of prefix names and prefix symbols to form the names and symbols of the decimal multiples and submultiples of SI units, ranging from 1012 to 10–12. Prefixes for 10–15 and 10–18 were added by the 12th CGPM (1964, Resolution 8), for 1015 and 1018 by the 15th CGPM (1975, Resolution 10), and for 1021, 1024, 10–21 and 10–24 by the 19th CGPM (1991, Resolution 4). Table 5 lists all approved prefix names and symbols.

Table 5. SI prefixes

Factor Name Symbol
Factor Name Symbol
101 deca da
10–1 deci d
102 hecto h 10–2 centi c
103 kilo k 10–3 milli m
106 mega M 10–6 micro µ
109 giga G 10–9 nano n
1012 tera T 10–12 pico p
1015 peta P 10–15 femto f
1018 exa E 10–18 atto a
1021 zetta Z 10–21 zepto z
1024 yotta Y 10–24 yocto y

Prefix symbols are printed in roman (upright) type, as are unit symbols, regardless of the type used in the surrounding text, and are attached to unit symbols without a space between the prefix symbol and the unit symbol. With the exception of da (deca), h (hecto), and k (kilo), all multiple prefix symbols are capital (upper case) letters, and all submultiple prefix symbols are lower case letters. All prefix names are printed in lower case letters, except at the beginning of a sentence.

The grouping formed by a prefix symbol attached to a unit symbol constitutes a new inseparable unit symbol (forming a multiple or submultiple of the unit concerned) that can be raised to a positive or negative power and that can be combined with other unit symbols to form compound unit symbols.

Examples: 2.3 cm3 = 2.3 (cm)3 = 2.3 (10–2 m)3 = 2.3 x 10–6 m3

1 cm–1 = 1 (cm)–1 = 1 (10–2 m)–1 = 102 m–1 = 100 m–1

1 V/cm = (1 V)/(10–2 m) = 102 V/m = 100 V/m

5000 µs–1 = 5000 (µs)–1 = 5000 (10–6 s)–1 = 5 x 109 s–1

Similarly prefix names are also inseparable from the unit names to which they are attached. Thus, for example, millimetre, micropascal, and meganewton are single words.

Compound prefix symbols, that is, prefix symbols formed by the juxtaposition of two or more prefix symbols, are not permitted. This rule also applies to compound prefix names.

Prefix symbols can neither stand alone nor be attached to the number 1, the symbol for the unit one. Similarly, prefix names cannot be attached to the name of the unit one, that is, to the word "one."

Prefix names and symbols are used with a number of non-SI units (see Chapter 4), but they are never used with the units of time: minute, min; hour, h; day, d. However astronomers use milliarcsecond, which they denote mas, and microarcsecond, µas, which they use as units for measuring very small angles

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