Sim's Electrical Companion
System Internationale de units
International system of units (SI Units)
SI units have been developed for the global scientific community to communicate within a framework that is internationally recognised*. The seven base units are derived from invariant constants in nature. Scientists are constantly working on defining the seven base units as finitely accurate as possible and historically they have been redefined many times.
The UK adopted the metric system in the late 1970's though we still hold dear some of our imperial measurements.
Some amusing examples are so common place you may not even think about it. We still count motorway distances in miles, order a pound of cheese, Measure our height in feet and signs displaying entrance 100 yards are still abundant.
* Burma (Myanmar), Liberia and the United States of America have not yet adopted the metric system.
Metric Prefixes
As well as System International a great deal of work has been done to ensure that scientists and engineers are able to work within a structured well defined system of measurement. Historically we had many different weights and measured used by different communities. The biggest problem with this was recreating anything in design and construction in an area where a different measure was used was made significantly more difficult.
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One simple example is the metre that is used worldwide for length but it is a relatively recent event. Originally it was a division of the circumference of the earth to make it 40 million metres. It now defined as 1/299,792,458 of the distance light travels in a vacuum in one second.
In Egypt the standard measurement was the cubit. A cubit was literally based on the fore arm length from elbow to middle finger tip of the current Pharaoh. A bizarre method of measurement with obvious problems when there was a change of management mid construction!
Previously in the UK we used the imperial standard. It was an ungainly method of measurement requiring the user to have in depth knowledge of fractions which brought with it more scope for errors. A good example is the $125 million Mars orbiter mission which was calculated to a very high degree of accuracy but was lost due to a mixture of imperial and metric calculations which ended in disaster.
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The far more simplified method of measurement using decimal base 10 set of numbering has revolutionised international cooperation. By adopting the same system worldwide construction and trading has become a far simpler affair.
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With the base 10 system calculations are far simpler. When working with very large or very small numbers metric prefixes are used to simplify calculations.
The list of prefixes I have tabulated are the most commonly used in electrical science. Although there are more for example deca x10 e.g.. Decade (10 years). I have omitted them for clarity as they are not regularly used in electrical science. Great care must be taken when you are working with mixed prefixes. Common examples where the prefixes are used are in power calculations, capacitance and resistivity.
