The SI System

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International System (SI)

In 1875, the &ldquo;Metre Convention&rdquo; mandated the establishment of a permanent International Bureau of Weights and Measures andthe Decimal System for working out related units.

The basic 7 units of measurement are as given in the table below.

All other units used by scientists are derived from these 7 units.

For example the unit of measure for angles is not degrees but radians which is based on length.

Different magnitudes were also needed to specify the exact value of these units. They were worked out as per the decimal system and given standard names. The current nomenclature is given below.

The SI is intended to be an evolving system; units and prefixes are created and unit definitions are modified through international agreement as the technology of measurement progresses and the precision of measurements improves. For example, the most recent derived unit, the katal, (an unit for measuring catalytic activity of enzymes) was defined in 1999.

We have to be careful about using the correct case (upper/lower) and they could mean different units.

As of now the SI has been adopted by all countries except the United States, Liberia and Myanmar. In US for some reason, milk is measured in gallons while soft drinks are measured in ml!

A recent addition to really large & really small numbers is googol & googolplex.

A googol is defined as one followed by one hundred zeros.

1 preceded by a googol zeros is a googolplex. If it’s difficult to comprehend how long that number would be, it’s because writing it out would result in a figure so large that if it were written down and published in a volume series of books, it would weigh more than the whole globe.

Non Decimal Systems

Many other unit systems have developed over time and do not follow the decimal system. They have got so entrenched in our life &amp; language that they have not been changed. Some examples are given below.

Time – Second, Minute, Hour, Day, Week, Month, Year. Higher units like Century &amp; Millenium follow the decimal pattern.

Byte – Due to technical reasons, the units for Information follow a system based on 8. 1 Byte = 8 Bits. 1 Kilobyte = 1024 bytes.

Distance – Inter-stellar distances use the &ldquo;light year&rdquo; to measure distances between stars &amp; galaxies. A &ldquo;Light Year&rdquo; is the distance travelled by light in one year.

Angles – The unit of angle measure is Radians in the SI system. It is not a basic unit but a derived unit.

Easy Conversions from British to Metric

Due to cultural & social reasons, we continue to use the British system still for many measurements. There are some approximate but easy ways to convert one into another for some measurements.

Mile to km - 1 mile = 1.6 km

Meter to feet - 1 m = 3.3 feet. Converting m to feet - multiply by 3 & add a tenth. 3 m = 9 + 0.9 feet = 9.9 feet

Kg to Pounds - 1 kg = 2.2 lbs. Converting kg to pounds - multiply by 2 and add a tenth. 3kg = 6 + 0.6 = 6.6 lbs.

1 sq m = 10 sq feet.

We will now look at the course which measurement may take in the future.

< 24.5 International System of Measurements | Topic Index | 24.7 Measurement – The Future >