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  Measurement is the art or technique of measuring, based upon pre-defined standards: it is a form of the ‘abstraction’ of reality vital to all scientific and technological activity. It has evolved into a science that has great consequences upon the life of human beings from mathematical theories to the size and weight of goods sold in supermarkets.

The theory of measurement was first studied by the Greek mathematicians Eudoxos of Cnidos and Thaeateros. The theory is based on the use of numbers to represent different objects and physical phenomena. Any measurement theory must involve three basic concepts, those of error, representation and uniqueness.

Errors will always be present; even though scientific experimentation techniques may improve the degree of accuracy the results obtained can always be subdivided to the extent that they are no longer accurate. A kilogram of fruit bought in a supermarket, for example, may actually weight 1.05 kg. This mistake in the degree of accuracy may not matter when fruit is being weighed, but if platinum were measured on the same scales, the error could be costly. Human error may always be present but modern technology can be used to improve accuracy of measurement and identify gross error.

Representation is really the assignment of numbers to such measurements, while uniqueness is the representation chosen for different objects. (Electric current, for example, is not measured in litres, nor is water measured in seconds, and it is this knowledge of the basic framework of measurements that leads to uniqueness.)

Measurement of any object or phenomenon is carried out by comparison. This leads to the need for a reference system from which all measurements are derived. The earliest measurements were based on four standards, those of mass, volume, length and area. The standards first appeared in ancient Mediterranean countries and were based upon what humans saw around them. The first linear measurement, the Egyptian cubit, was based on the measurement from the elbow to the fingertip, while the inch was based on the width of a thumb. These highly inaccurate measurements led to confusion throughout the Western world. The need for standardization was essential if the Industrial Revolution was to succeed. Specialized tools such as borers and lathes could never have worked if measurements were made to such a high degree of inaccuracy.

The adoption of the British Imperial, US Customary and metric system in Europe solved this problem. Nowadays, of course, the standard system in most countries is the SI system (Système d\'Unités Internationale) which by standardization has made many parts and components interchangeable throughout the world. This standardization has transformed industry\'s way of thinking as more and more goods are built on flow-line techniques, so reducing the cost of goods, since assemblers like car manufacturers can rely on the precision of manufacture of components from many different companies and countries.

In the SI system the basic references are the metre (m) for length, the kilogram (kg) for mass, the second(s) for time, the ampere (A) for electrical current, the kelvin (K) for temperature and the candela (cd) for light intensity. The metre was originally defined as the length of a metre bar kept in Paris and the kilogram is the mass of a metallic block maintained there also. While the reference for the kilogram has been unchanged for nearly 200 years the standard for length is related to the wavelength of vibrations of the rare element krypton 86. Similarly the base of time the second is defined relative to the frequency of radiation produced from Caesium 133 under certain conditions.

The English speaking public has difficulties with the units for force, since these are the technically correct ways of expressing weight (the force of gravitational attraction on a body). The reference unit is the newton (N) defined as the force which when applied to a 1 kg mass produces 1 metre per second squared acceleration. This is used in the definition of ampere and the kelvin for temperature is derived from the difference between absolute zero temperature and the freezing point of water.

Instruments to make measurements have required to be developed as technological advances are made. Round-the-world navigation by sea depended on the invention of a reliable time piece, the chronometer. The diameter and lengths of machine components have successfully been measured using wooden then steel rules, vernier calipers, micrometers, comparators using slip gauges and finally lasers. Force or weight is measured by spring balances where displays are mechanical pointers or electrically generated numbers. High technology is available at low cost for time measurement where the markets are stocked with watches whose timekeeping is based on the frequency of oscillation of a crystal of quartz. AA

Further reading T.F. and M.B. Gilbert, Units and System of Weights and Measure, their Origin, Development and Present Status.



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