The Metric system

WHY  CAN'T  WE  EVER  GET  IT  RIGHT?

Isn't it odd that Americans and even some Europeans - some of the most technologically sophisticated people on earth - are so sloppy and at times ignorant in their communications between themselves and others?

Since 1866 the so-called “metric system” of units (weights and measures) has been legal for trade in the United States. In the EU it is now the only system that can legally be used for trade.  In 1960 an international group formalized the units used all over the world.  The international “General Conference on Weights and Measures” met in Paris and agreed the coherant system of units (based on the meter, kilogram, second, ampere, Kelvin,mole and candela) the “International System of Units.”  The Conference also established the abbreviation SI (Système International d'Unités) as the official abbreviation, to be used in all languages. 

SI  UNITS  AND  STANDARD  USAGE

There are seven basic SI “base units,” these are:

The SI units are used to derive units of measurement for all physical quantities and phenomena. 

Note particularly that there is no SI unit for volume.  Volume is a cube function of length, as in the case of a solid or hollow cube that has a length, a width and a height.  You will also notice—even if you read every document you can find that even mentions SI units—that there is no such thing as a litre (liter) in the SI system of units.  This is because the proper way to describe volume is in terms of the SI base unit (meter) that volume is derived from.  For example, the proper way to describe one (liter) of water is to use one of the two terms: 1000 cubic centimeters (1000 cc) or one cubic decimeter (1 dm³).  Which is the preferable term to use is detailed later on in this missive.

Note also that even those well-intentioned but incorrect people who use “metric” names (even if these names are made up ones such as “litre,”( “liter”) which comes from another bastardized system that SI was intended to replace), often get the capitalization use rule wrong.  The rule is: if, and only if, the unit is named after a person, use upper case for the letter designator.  There is no record of a scientist named Liter 
(or even Litre) for whom a unit was named, so  stop capitalizing this unit—which, of course, you shouldn't be using in the first place.

The SI derived units are listed here with their derivative units:

Supplementary units:

 SI PREFIXES

The names of multiples and submultiples of any SI unit are formed by application of the prefixes: (where possible a prefix representing 10 raised to a power which is a multiple of three should be used)

Some examples: 

Always use fewer than 1000 units with SI prefixes; “ 1000MGS ” is advertising hyperbole and should be written “ 1 g ” only.  If you insist on using a thousand milligrams, write: “ 1000 mg ” to be consistent.

SI prefixes and units should be written together and then set off by a space (single space in print) from their numerators.  For example; use the form “ 35 mm ” instead of  “ 35mm ” and “ 1 kHz ” instead of “ 1k Hz .”

The creeping scourge of euro-abbreviation such as the use of  “ 4k7 ” for 4700 ohms or “ 3k3 ” for 3.3 k is also, strictly speaking, non-SI gibberish that should be avoided unless nothing else physically fits a small space and is still readable, such as on the body of a tiny electronic component itself.

When writing use only standard SI formats and be consistent. 

Never combine SI prefixes directly, for example, write 10^-10 farads (0.0000000001 F) as 100 pF instead of 0.1 micro-microfarads (uuF). 

The rule for unit names written longhand is that the name is all lower case, but when abbreviating, the first letter is upper case if the unit is named after a person and lower case if it is not. 

Examples:

V = volt for Count AlessandroVolta, 1745-1827, the Italian physicist who invented the first electric battery (1800). 

F = farad for Michael Faraday, 1791-1867, the British physicist and chemist who discovered electromagnetic induction (1831) and proposed the field theory later developed by Maxwell and Einstein. 

T = tesla for Nicola Tesla, 1856-1943, Serbian-born American electrical engineer and physicist who discovered the principles of alternating current (1881) and invented numerous devices and procedures that were seminal to the development of radio and the harnessing of electricity.  

Letter m = meter, s = second, rad = radian, and so on.

In addition to the correct upper and lower case, prefixes and combinations, there is also a conventional text spacing for SI units and abbreviations.  Write 20 Hz, rather than 20Hz.  Write 20 kHz, rather than 20k Hz, and so on.  Always separate the numerator of a unit from its prefix and/or unit name, but do not separate the prefix and name. 

Definitive pamphlet describing SI units, conversions between SI units, older CGS and MKS units and units outside the SI system of units are available in a number of documents including:

NASA Publication SP-7012, (1973).  Inquire to the U.S. Government Printing Office in Pueblo, Colorado or in Washington, D.C. for this and other US publications about SI units, their use and history.

National Bureau of Standards publication 330, (1977) for details on SI usage.

The IEE and IEEE also have a selection of pamphlets.