2.2.1 - Origins & Development of SI

2.2.1

Origins & Development

of The SI Prefixes

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INTRODUCTION

Even though the scientific community often deals with quantities greater than a trillion on a regular basis , they rarely use the illion names above a trillion. Perhaps part of this adversion has to do with the fact the there exists 2 standard systems, the short and long scale, so that any use of these terms on an international basis has some level of ambiguity. Instead the scientists have championed the use of scientific notation. They do however have another system they make frequent use of, the standard set of SI prefixes. For example, scientists will often speak of micrometers and nanometers. These terms are formed by combining an SI prefix, with a unit suffix in order to form a new unit. A micrometer = 10^-6 meters, while a nanometer = 10^-9 meters. This is because micro stands for a multiple of 10^-6 , and nano for a multiple of 10^-9. These same prefixes can then be applied to other units, for example , a nanosecond = 10^-9 seconds.

The SI prefixes are mainly used in science, but they are also used in computer technology. Perhaps the common use of such terms as gigabyte has lead to their popularity among ordinary people who find this sort of trivia interesting. The SI prefixes are linked to the subject of large numbers, in that they are used to express very large and very small values that occur in science. So what are the SI prefixes and how do they work ? Well let's first discuss their origins and development, before we talk about the system as it is used today...

Origin and Development of the SI prefixes

The SI prefixes were created as part of a more sweeping campaign in the scientific community, the invention of the metric system. Around 1793 Louis XVI of france commissioned the creation of the metric system[1] . The conventions of the system were first conceived by a group of french scientists. On August 1 , 1793 the National Convention adopted the new unit of length, the "metre". The metre was originally defined so that the circumference of the earth would be exactly 40,000,000 metres. On December 10 , 1799 the metric system was definitively adopted in france[1].

The original base units used were the metre, gram, second, ampere , kelvin , and the candela. Most people recognize the metre as a measure of length, the gram as a measure of mass, and the second as a measure of time.

These six "base units" could be combined with a series of SI prefixes to form new units which were multiples of the base units.

The original SI prefixes that were officially adopted in 1795 , were deca, hecto, kilo, myria , deci , centi , milli, and myrio. The table below shows the multiple the prefix represents followed by the prefix, and lastly each prefix has a "prefix symbol" ...

Multiplier SI Prefix Prefix Symbol ( SI Prefix Table of 1795 system )

10,000 myria- ma

1,000 kilo- k

100 hecto- h

10 deca- da

0.1 deci- d

0.01 centi- c

0.001 milli- m

0.0001 myrio- mo

The prefixes were derived from greek and latin numbers. For example deca- is an adaption of deka, which is greek for ten, hecto is an adaption of ekato which is greek for a hundred, and kilo is an adaption of chilia which is greek for a thousand. myria, and myrio are derived from myriad, which was greek for ten thousand.

Likewise deci , centi , and milli, are derived from the latin for tenth, hundredth, and thousandth respectively.

One can now construct many useful units by combining prefixes with base units. For example, rather than speak of a thousand metres, one can speak of kilometres. A kilometre ( symbolized as km ) is similiar to the mile, although it is actually shorter ( a kilometre is about 0.62 miles ).

Instead of a thousand grams, one can use the kilogram ( symbolized kg ). The kilogram is comparable to pounds. On earth a kilogram is equivalent to about 2.2 pounds ( Note: where kilogram is a measure of mass, pounds is actually a measure of weight. Therefore , while our mass in kg remains constant, our weight in pounds will change depending on which gravitational field we are under ).

The metric system, along with it's SI prefixes, eventually went international. In fact SI basically stands for "System International". The metric system is now one of the most widely used systems of measurement.

As science grew, so did the need for bigger and smaller numbers, and as a consequence a need for more prefixes. Prefixes were slow to come forth however...

In 1960[2] myria and myrio were dropped from the list, but 6 completely new prefixes were added ! They were ...

Mega- for 10^6 : Mega is derived from the greek word for "big" [3]

Giga- for 10^9 : Giga is derived from the greek word for "giant"

Tera- for 10^12 : Tera is derived from the greek word for "monster"

To parallel these, 3 new "small" prefixes were added as well ...

micro- for 10^-6 : micro is derived from the greek word for "small"

nano- for 10^-9 : nano is derived from the greek word for "dwarf"

pico- for 10^-12 : pico is derived from the greek word for "tiny bit"

The need for the tiny prefixes was in the growing field of particle physics, which deals with very small scales of space and time. The need for the large prefixes would come later when computers contained millions and billions of bytes, and lead to the well known terms megabyte and gigabyte.

Some further additions followed. In 1964 two more SI Prefixes were added to the standard roster. They were ...

femto- for 10^-15 : femto is derived from the Dano-Norwegian word for "fifteen"

atto- for 10^-18 : atto is derived from the Dano-Norwegian word for "eighteen"

Oddly this meant there were more "small scale" SI prefixes than large ones. The corresponding large SI Prefixes were added in 1975 ...

peta- for 10^15 : peta is derived from the greek word for "five"

exa- for 10^18 : exa is derived from the greek word for "six"

Lastly as late as 1991 four more SI prefixes were added. These being zetta- , zepto- , yotta- , and yocto- ...

yotta- for 10^24 : yotta is derived from the latin word for "eight"

zetta- for 10^21 : zetta is derived from the latin word for "seven"

The small scale SI prefixes are ...

zepto- for 10^-21 : zepto is derived from the latin word for "seven"

yocto- for 10^-24 : yocto is derived from the latin word for "eight"

This brings us up to speed on the modern SI prefixes. The system currently supports 20 standard SI prefixes.

Modern Standard

In the next article we will go over the system as it is today, and how it is used.

NEXT>> 2.2.2 - Modern Standard SI

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Source material :

[1] http://en.wikipedia.org/wiki/SI : This wikipedia article is where I got some of the information on the origins and use of the SI prefixes.

[2] http://www.mrob.com/pub/math/largenum-2.html : This webpage is part of Robert Munafo's large number site. In the middle of the page under "SI prefixes", Munafo discusses some of the history of the SI prefixes, which is the source of my dates relating to the introduction of the SI prefixes.

[3] http://lamar.colostate.edu/~hillger/si-prefixes.html : I learned of the etymology of the SI prefixes from this site.