The S.I. Standards

The standard of a unit is the method used to define the unit. A standard should satisfy two properties:

• It should precisely define the amount, so there is agreement about the size of the unit.

• It should be invariable and easy to reproduce worldwide.

Let us examine closely the development of the standard for length:

Early units, based on physical dimensions of parts of the human body, were poor standards, because they varied from person to person. For example, the Egyptian cubit (the distance from a man's elbow to the end of his middle finger),or the 15th century definition of a yard (the distance from the tip of the nose to the end of the index finger).

A more recent definition of the yard (a non-metric unit) was as the distance between two marks on a particular bronze bar kept at 62 degrees Fahrenheit in the National Physical Laboratory in England. This standard does not quite satisfy the second requirement above --- it is not easily reproducable world-wide --- but for a while, this was the best definition everybody could agree upon.

In 1974, the precise definition of the metre was based upon radiation from the krypton-86 atom (which has nothing to do with Superman).:

"The metre is the length equivalent to 1 650 763.73 wavelengths in a vacuum of the radiation corresponding to the transistion between the levels 2p and 5d of the krypton-86 atom." (Alward and Findlay, 1994)

It is not too important what this definition means (without some chemistry or physics background, it may look like gibberish), but notice how precise it is. Observe also that this definition is in terms of some physical quantity that could be measured in any reasonably well-equipped laboratory at any location in the world.

In 1983, the definition of the metre was revised again, in order to take advantage of developments in laser based methods of maintaining length standards.

On the next page, we have a table showing the definitions of the seven SI units, together with the date of the most recent revision.