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Many SI prefixes predate the introduction of the SI in 1960. They can be applied correctly to many non-SI units. As part of the SI system they are officially determined by the Bureau International Des Poids Et Mesures . OVERVIEW SI defines a number of is 1000 Metre s, the ''kilo''gram is 1000 Gram s, a ''kilo''watt is 1000 Watt s, and so on. The prefix ''milli'' subdivides by a thousand, so a ''milli''metre is one-thousandth of a metre (1000 millimetres in a metre), and a ''milli''litre is one-thousandth of a Litre . The prefixes are never combined; a millionth of a kilogram is a ''milligram'', and not a 'microkilogram'. The ability to apply the same prefixes to any SI unit is one of the key strengths of the SI, since it considerably simplifies the system's learning and use. The following SI prefixes can be used to prefix any of the above units to produce a multiple or submultiple of the original unit. This includes the degree Celsius (e.g., “1.2 m°C”); however, to avoid confusion, prefixes are ''not'' used with the time-related unit symbols min (minute), h (hour), d (day). They are not recommended for use with the angle-related symbols ° (degree), ′ (minute of arc), and ″ (second of arc) {Link without Title} , but for astronomical usage, they are sometimes used with seconds of arc. See Also: Non-SI unit prefixes Examples:
Prefixes cannot be combined: for example 10−9 metre must be written as 1 nm, not as 1 mµm. The prefix always takes precedence over any exponentiation; thus "km&2" means ''square kilometre'' and not ''kilo–square metre''. For example, 3 km&2 is equal to 3 000 000 m&2 and ''not'' to 3000 m&2 (''nor'' to 9 000 000 m&2). Thus the SI prefixes provide steps of a factor one million instead of one thousand in the case of an exponent 2, of a billion in the case of an exponent 3, etc. As a result large numbers may be needed, even if the prefixes are fully used. Prefixes where the Exponent is divisible by three are recommended. Hence "100 m" rather than "1 hm". The Obsolete Prefixes ''myria-'' and ''myrio-'' were dropped before SI was adopted in 1960 , probably because they do not fit this pattern, no symbol was available (M, m and µ already being used), and were rarely used anyway. Double prefixes such as those formerly used in ''micromicrofarads'' (picofarads), ''hectokilometres'' (100 kilometres), and ''millimicrons'' or ''micromillimetres'' (both nanometres) were also dropped with the introduction of the SI. The Kilogram stands out among all SI Base Unit s as the only one that has a prefix. It is derived from the mass of an actual object. The Gram is defined as 1/1000 of this object's mass. Though in principle legal, most combinations of prefixes with quantities are very rarely used, even in a scientific or engineering context:
The reason various fields have develop their own non-SI scales is because of the problems posed by calculating very large or very small numbers on a computer accurately and efficiently. 1 angstrom is a lot easier for a computer to use in calculations than 1 × 10-10 as Truncation Errors and Rounding errors can occur losing accuracy in the calculation. † the United Kingdom, Ireland, Australia and New Zealand previously used the Long Scale number name conventions, but have now at least partly switched to the Short Scale usage. Note in particular that above a million and below a millionth, the ''same'' name has different values in the two naming systems, so ''billion'' and ''trillion'' (for example) have unfortunately become potentially ambiguous terms internationally. Using the SI prefixes can circumvent this problem. PRONUNCIATION The accepted English pronunciation of the initial G of ''giga'' was once soft, (like ''giant''), but now the hard pronunciation, (like ''giggle''), is significantly more common. However, both pronunciations are likely to be understood by most English speakers, though the second is likely to be preferred. Since 'kilometre' is a combined word, it should logically be pronounced with a stress on the first syllables of the composite words (k''I''lom''E''tre), as in the other units, such as 'kilogram', but it is not uncommon in the English language to put the stress on the 'o' (kil''O''metre). USE OUTSIDE SI The symbol "K" is often used to mean a multiple of a thousand, so one may talk of "a 40K salary" (40,000), or the Y2K problem. Note that in these cases an upper case K is often used, although it should be noted that using an uppercase K is never correct when writing under the rules of the SI. Also, it is often used as a prefix to designate the Binary Prefix kilo = 210 = 1024. Non-SI units
Computing See Also: Binary prefix The prefixes K and greater are common in Computing , where they are applied to information and storage units like the Bit and the Byte . Since 210 = 1024, and 103 = 1000, this led to the SI prefix letters being used to denote "binary" powers. Although these are incorrect usages according to the SI standards it seems common to apply base 10 prefixes, when relating to computers, as follows: ; K:= 210 = 1,024 ; M:= 220 = 1,048,576 ; G:= 230 = 1,073,741,824 ; T:= 240 = 1,099,511,627,776 ; P:= 250 = 1,125,899,906,842,624. These prefixes, however, usually retain their powers-of-1000 meanings when used to describe rates of data transmission ( Ethernet runs at 10,000,000 bit/s, not 10,485,760 bit/s. The problem is compounded by the fact that the units of information (the Bit and the Byte ) are not part of SI, where the bit, byte, Octet , Baud or Symbol Rate would rather be given in Hertz . Although some use "bit" for the Bit and "b" for the Byte , "b" is often used for bit and "B" for byte instead. (In SI, B stands for the Bel , although its sub-unit, the decibel ("dB"), is almost universally used instead, preventing confusion between the symbols.) It is recommended by several standards bodies to use bit and '''B''' to keep the units very distinct, as in ''kbit'' or ''MiB''. French-speaking countries often use "o" for "octet", nowadays a synonym for byte, but this is unacceptable in SI because of the risk of confusion with the zero. Consequently, the International Electrotechnical Commission (IEC) adopted new Binary Prefix es in 1998 , formed from the first syllable of the decimal prefix plus 'bi' (pronounced 'bee'). The symbol is the decimal symbol plus 'i'. So now, one Kilobyte (1 kB) equals 1000 bytes, whereas one Kibibyte (1 KiB) equals 210 = 1024 bytes. Likewise Mebi (Mi; 220), Gibi (Gi; 230), Tebi (Ti; 240), Pebi (Pi; 250), and Exbi (Ei; 260). Although the IEC standard does not mention them, the sequence can be readily extended to Zebi (Zi; 270) and Yobi (Yi; 280). The adoption of these prefixes has been very limited. PROPOSED CHANGES Extension Continuing backwards in the alphabet, after ''zetta'' and ''yotta'', proposals for the next large number include ''xenta'' and ''xona'' (among others), the latter as an alteration of the , and the next ''small'' number would also start with an ‘x’. Preserving the rule on abbreviating the prefixes (a ), the symbol for “micro”. However, even some official prefixes may not be understood by all readers, let alone extrapolations of them, so giving an explanation is advisable when using them in communication (as opposed to using them in notes for oneself). Another proposal for xenta/xona is ''novetta'', from the Italian . This does not have the convenience of backward alphabetic order. Harmonization There are also proposals for further harmonization of the capitalisation. Therefore the symbols for deka, hecto and kilo would be changed from ‘k’ to ‘K’, from ‘h’ to ‘H’, and from ‘da’ to ‘D’. Likewise some lobby for the removal of prefixes that don't fit the 10±3''n'' scheme, namely hecto, deka, deci and centi. The CGPM has tabled its decision on both matters for now. An unsolved (and maybe unsolvable) issue is the application of prefixes to units with exponents other than ±1. The prefix is always applied before the exponent. This eventually lead to the introduction of special units for area and volume without exponents in the original Metric System :
Of these the litre and the hectare are the most ubiquitous in common use: Litre designations are sometimes used to differentiate a volume of fluid (as opposed to a gas, or solid which are usually designated as cubic volumes). Hectares are widely used as a metric alternative to the acre (approximately 2.5 acres to the hectare). POPULAR CULTURE Several prefixes have made appearances in popular culture, specifically in America, in one fashion or another which caused them to stand out. The original pronunciation of giga-watt was one such example, in the '' Back To The Future '' movies. This usage predated the common occurrence of gigabytes and gigahertz in common American usage, but does not seem to have affected much the fact that Americans seem to prefer the hard G. Additionally, an American tape backup hardware company has been named Exabyte since well before it became possible that anyone would ever need to back up that much data; how the march of Moore's Law might affect their corporate name is unknown, but see 20th Century Fox . SEE ALSO
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