For Good Measure.

Units and Standards

A Bit of History

Accurate measurements of physical quantities such as distance, time, weight, temperature, electrical power, and others are essential for fair trade and commerce. Any successful trading of objects (or ideas) requires that the participants have a good understanding of what it is that they are exchanging. The history of measurement predates written records. The first properly documented example of a unit of measure is the cubit as it was developed in ancient Egypt. The Egyptian cubit was defined as the length between the elbow and the fingertips of (someone's) arm. Most early length units were based on the size of human body parts and other familiar everyday objects. It is interesting to note that the ancient Egyptian's soon discovered a need for what we would nowadays called a standard. By 2000 BC the cubit was represented by an official permanent object, a piece of marble, the royal cubit. In contrast the English units of measure were not standardised until the 13th century. Even then there were often diffferent standard for units representing the same physical idea in different contexts. For example the gallon, a unit representing the physical idea of volume, was not standardized in England until the early 1800's! Until then the wine gallon was different from the beer gallon.
The United States system was adopted from the English and standardised in 1836. The standards agreed closely but were not identical. And, as in England, there was a wine-gallon of 231 cubic inches and a dry gallon of 269 cubic inches. There was also a small discrepancy between the English inch and the American inch until 1959 when both standardised to exactly 2.54 centimeters. (The metric system is the legal system in both countries now even though the United States uses both systems in everyday life.)

"For all people, for all time."

The French Revoluiton brought about many changes, amongst them the metric system of units. Adopted in France in 1799, "for all people, for all time", the metric system has two important characteristics. The standards are based on physical events which are as permanent as possible, and the units group and subdivide in multiples of ten. No more yards, feet, and inches. The original "meter" was to have been a one ten-millionth of the distance from the pole to the equator. This is a standard based on science and technology. The idea of such a standard ultimately led to the (almost) universall system of units called the SI system.

The International System

All physical quantities are based on three fundamentals concepts: distance, time and mass.

The three fundamental quantities are measured in terms of the three basic units of measure which in International System of units are: meter, second and kilogram. The International System or the SI system is often refered to as the metric system. Human interactions, in industry and commerce among others, depend on measuring devices based on these units adopted in 1946.

The definitions of the three fundamental units are:

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.

The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram. These definitions were take from the home page of the National Physical Laboratory in Great Britain.

The Keepers of the System

Every country maintains an instituion responsible for the proper functioning of measuring devices and for research and development in this area. In the United States this institution is the National Institute of Standards and Technology (formerly called The Bureau of Standards).

NIST is administered by the Department of Commerce. NIST mainatains several laboratories. The one pictured here is in Boulder, Colorado.

YOU CAN GET ACCURATE TIME FROM NIST.

Some Rules To Remember

Use only one prefix. You may not use millimicro for 10^-9. The correct prefix is nano.
When a unit carries a person's name (e.g. newton) use lower case when you spell it out, use capitals (N for newton) when you abbreviate.
Don't pluralise abbreviations. 10 meteres is abbreviated as 10 m, not 10 ms.

More info:
1. 2. 3. 4. For extra credit answer the following questions.

1. What is the legal status of the metric system in the U.S.?

2. What agencies are responsible for the accuracy of the measuring devices in the stores across the country?

3. Notice that the definition of the kilogram is in terms of a physical object (an artifact) whereas the meter and the second are defined in terms of fundamental physical processes. An effort is underway to define the kilogram in a similar way? What is being proposed.