International System of Measurements

< 24.4 Story of Measurement 2 | Topic Index | 24.6 The SI System >

The Magna Carta written in 1215 was the earliest national document which talked of the need for standards to measure units like length, area, volume, weight etc.

The efforts to arrive at an internationally accepted system of units started with moves by the French government in late 18thcentury to adopt the metric system of units. In 1875 the General Conference on Weights &amp; Measures was set up in Paris. This conference established the Meter Convention.

The mathematician Joseph Lagrange who was a member of a committee at the Academy of Sciences, Standardized Weights and Measures, played a significant role in the adoption of this new system. He was able to essentially convince the committee to adopt the new unit system of measurement that used meters and kilograms, i.e. the metric system. The one used at the time was not conducive for trade, likely because it was not universal. It was approved by the commission; and now the metric system is widely used as the only system of measurement in the majority of the world.

Length  - The Meter Convention called the unit of length as metre. Metre was defined as equal to one ten-millionth of the distance between the North Pole and the Equator. In 1889 this organization established the International Prototype Metre as the distance between two lines on a standard bar composed of an alloy of 90% platinum and 10% iridium, measured at the melting point of ice.

From 1983 the Meter has been defined in terms of the distance travelled by light in a particular amount of time. (1/ 299792458thof a second)

Astronomical Distances

Since distances used in astronomy were huge, using the "metre" as the unit gave rise to very big numbers. Hence two different units to measure astronomical distances were evolved.

Light Year

A "light year" is the distance travelled by light in vacuum in a year.

Parsec

Most measurements in astronomy involved calculating angles subtended by the object whose distance is to be measured. A "parsec" is the distance from which the "distance from the Earth to the Sun" can be seen at an angle of 1 angular second. Parsec is a short form of parallax-second.

The development of units for other properties was similar to that of the &ldquo;metre&rdquo;. They all started with &ldquo;local&rdquo; standards which slowly developed into &ldquo;international&rdquo; standards.

Weight – In 1889, the Meter Convention also defined the mass of a kilogram as that of cylinder of platinum and iridium kept in France. It is called the international prototype kilogram and has the same mass as one litre of water at the freezing point.

Second– Previously it was defined as a fraction of the duration of a day (of 24 hours). From 1967 it has been defined in terms of the time taken for a &ldquo;change of state&rdquo; in thecaesium-133 atom. The measurement of time still uses historical units like minute, hour, day, week, month year etc.

Angle – Angle is another measurement which uses the historical units like second, minute &amp; degrees. For scientific purposes an angular unit called &ldquo;radian&rdquo; is used. A radian is defined as the angle subtended as the centre of a circle, by an arc whose length is equal to the radius of the circle.

Radians are more &ldquo;geometrical&rdquo; in concept. They make mathematical expressions involving angles, much more simpler and elegant.

Temperature– The Kelvin scale was defined in terms of the freezing and boiling points of water at sea level. The range between the freezing &amp; boiling points of water was divided into 100 degrees.

Derived Units

Many other units were defined which depended on the above units. Area, Volume, Density, Speed are some of the units which are derived from the base units. The radian is also a derived unit.

Humans Need Small Numbers

From the standard unit of measure, smaller and larger units were also defined using &amp; extending the decimal system. There were 2 major reasons for this.

Let us now see the details if the International ST system.
 * 1) The human brain can visualize &amp; comprehend only 2 or at the most 3 digit numbers. Different magnitudes of units allow a property to be presented in smaller numbers for easy comprehension. For example, it would be difficult if we had to remember that our weight is 65,138 Gms. Remembering it 65 Kgs makes it easy for us. It also helps us to say that a recipe needs 5 Gms and the weight of a car is 4.5 Tonnes (1000 kgs). All these units are decimal multiples or fractions of the standard unit Gram.
 * 2) Advances in science &amp; technology have made it necessary to have units to express both extremely small &amp; extremely big magnitudes.

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