People have been measuring things for a very long time. The earliest measurement units known date back to five or six thousand years in Egypt which included length and weight and used a decimal system.

As time went on, units of length started making references to body parts so that normal people could make a rough estimate of the units. We are all familiar with feet and yards; a foot was obviously relating to someone's (rather large!) foot and a yard was measured from the fingertips to the shoulder. It seems that the people that standardized these units must have been giants since they are all a lot longer than the average person today.

Over time, more and more units were created for various purposes such as the acre for area. "Acre" is an old English term for "field" and was meant to be the area of land that could be ploughed in one day using a yoke and oxen. A farmer with seven acres of land could then immediately deduce that it would take a week to plough his farm.

After a while it became necessary to standardize all of these measurement units and so various methods were put into place to try to make sure that everyone was referring to the same quantity when quoting a number of a certain measure. For instance, a yard became defined as the length of a pendulum for the period of the swing to equal one second- here we see the scientists creeping into the subject to try to pit these units against the laws of nature.

In 1789, the French Revolution took place and after cutting the heads off of all the dignitaries decided to structure their society around knowledge and philosophy and, thus, were the first to adopt the metric system which was in the main, based around scientific observation and natural laws rather than the rather arbitrary units used beforehand.

The Celsius and Fahrenheit scales

The metric system adopted centigrade as the unit of temperature (now more commonly known as the Celsius scale).

Previously, by far the most common temperature scale in use was Fahrenheit. This was, again, based on the human body in part where 100 degrees would be the normal human body temperature and 0 degrees bizarrely was established as the "stabilized temperature when equal amounts of ice, water, and salt are mixed". The human body temperature was later found to actually be about 98 degrees, which added more confusion to the scale.

Celsius created a very logical scale based on water- 0 degrees was the freezing point and 100 degrees was the boiling point. Everything in between was carved into 100 degrees and this was the unit of temperature. A much more logical approach and easily recreated by experiment so that thermometers and other instruments could be calibrated.

Since then another scale closely related to Celsius has been used in the science world- Kelvin. The only difference with this scale is that although each degree is the same, it starts at "absolute zero" which is the coldest that anything in the universe can get! There are no negative Kelvin values as you cannot get any colder than this, not even in the flat I used to live in four years ago! This makes the freezing point of water 273.15K (warm by comparison) and the boiling point adding an additional hundred degrees to 373.15K. On an odd note, degrees Kelvin is not normally given in degrees and the circle symbol is not used, just the "K".

The Meter and its derivatives

The meter was originally defined by the French as the length between two marks on a platinum-iridium bar (which was designed to represent 1/10,000,000 of the distance from the equator to the north pole through Paris). The bar would be kept in Paris and additional bars could be made against this "master copy" for general use.

Since then it has been redefined against universal physics as the distance travelled by light in absolute vacuum in 1/299,792,458 of a second.

All other metric measurements for length, area and volume are derived from this unit. A kilometer is a thousand meters, a millimeter is a thousandth of a meter and so on. Area measurements can either be the square equivalents of the meter (a meter squared is the area covered by a square of one meter by one meter) or the commonly used land unit the hectare which is 10,000 square meters.

Similarly, the volume measurements are the cubic counterparts; a liter being 10 centimeters by 10 centimeters by 10 centimeters. Derivatives of this include the milliliter (a thousandth of a liter) which is one centimeter cubed.

Another unit used for extremely large distances is the Parsec and the Lightyear which are based on the speed of light in a vacuum.

The metric Mass and Weight measurements

Weight, or for the pedants out there, Mass is measured against the properties of pure water.

The Kilogram unit of weight (which is surprisingly the ISO unit rather than the Gram) is the mass of one liter of water. A gram then follows as a thousandth of a kilogram etc.

Note that a metric ton (or Tonne) is one thousand kilograms and should not be confused with the imperial/ English measurements of short and long ton.

Countries that have not yet caught up

Despite the fact that the future of measurements is clearly metric, there are several countries that seem to be clinging on to their old ways.

The United Kingdom may be metric officially but it took a recent law to force shop keepers from continuing to sell their goods in pounds and ounces "and use those damn French units!", all of the roads are still in Miles and have Miles-per-Hour speed limits and the weather forcasters still quote both Celsius and Fahrenheit. However, since the price of gas shot up during the first Gulf war they now buy their petrol in liters since it sounds a lot less expensive.

There are three remaining countries which have not adopted the metric system at all- Liberia, Myanmar and the United States. I'm pretty sure I know which country is going to be the last!