SI Units
THE INTERNATIONAL STANDARD SYSTEM OF UNITS GIVING A COMMON LANGUAGE BETWEEN countries and different branches of science
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SI units are formally defined using a known constant value. For example, the metre is defined in terms of the speed of light. It is "the distance travelled by light in a vacuum during a time interval of 1/299,792,458 of a second".
Until recently, the kilogram was defined by a physical object shown in Figure 1 - a cylinder made of 90% platinum and 10% iridium called the "International Prototype Kilogram" or IPK. Scientific laboratories have recently redefined the kilogram in terms of another fundamental constant h (Planck's constant). |
The video to the right is optional extra viewing.
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