Weights and Measures

Length, capacity, and weight can be measured using standard units. The principal early standards of length were the palm or hand breadth, the foot, and the cubit, which is the length from the elbow to the tip of the mid­dle finger. Such standards were not accurate and defi­nite. Unchanging standards of measurement have been adopted only in modern time.

In the English-speaking world, the everyday units of linear measurement were traditionally the inch, foot, yard and mile. In Great Britain, until recently, these units of length were defined in terms of the imperial standard yard, which was the distance between two lines on a bronze bar made in 1845.

In Britain units of weight (ounces, pounds, and tons) are now also derived from the metric standard — kilogram. This is a solid cylinder of platinum-iridium alloy main­tained at constant temperature at Sevres, near Paris. Cop­ies, as exact as possible, of this standard are maintained by national standards laboratories in many countries.

International System of Units is a system of meas­urement units based on the MKS (metre-kilogram-sec-

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ond) system. This international system is commonly re­ferred to as SI.

At the Eleventh General Conference on Weights and Measures, held in Paris in 1960 standards were defined for six base units and two supplementary units:

Length

The metre had its origin in the metric system. By in­ternational agreement, the standard metre had been de­fined as the distance between two fine lines on a bar of platinum-iridium alloy. The 1960 conference redefined the metre as 1, 650, 763.73 wavelengths of the reddish-orange light emitted by the isotope krypton-86. The metre was again redefined in 1983 as the length of the path travelled by light in a vacuum during a time inter­val of 1/299, 792, 458 of a second.

Mass

When the metric system was created, the kilogram was defined as the mass of 1 cubic decimetre of pure water at the temperature of its maximum density or at 4.0 °C.

Time

For centuries, time has been universally measured in terms of the rotation of the earth. The second, the basic unit of time, was defined as 1/86, 400 of a mean solar day or one complete rotation of the earth on its axis in relation to the sun. Scientists discovered, however, thAt the rotation of the earth was not constant enough to serve as the basis of the time standard. As a result, the second was redefined in 1967 in terms of the resonant frequency of the caesium atom, that is, the frequency at which this atom absorbs en­ergy: 9, 192, 631, 770 Hz (hertz, or cycles per second).

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