SI Units: Length
The definition of the meter (m), which is the international unit of length, was once defined by a physical artifact - two marks inscribes on a bar of platinum-iridium. Today, the meter (m) is defined in terms of constant of nature: the length of the path traveled by the light in vacuum during a time interval of 1/299, 792, 458 of a second. The Length - Evolution from Measurement Standard to a Fundamental Constant explains the evolution of the definition of the meter. Follow these changes over time in the NIST Length Timeline.
From the meter, several other units of measure are derived such as the:
FAQ: When did the metric redefinition of the inch occur?
In 1958, a conference of English-speaking nations agreed to unify their standards of length and mass, and define them in terms of metric measures. The American yard was shortened and the imperial yard was lengthened as a result. The new conversion factors were announced in 1959 in Federal Register Notice 59-5442 (June 30, 1959), which states the definition of a standard inch: The value for the inch, derived from the value of the Yard effective July 1, 1959, is exactly equivalent to 25.4 mm.
The conversion factor can be determined:
FAQ: How do I get a metric ruler?
Metric rulers are available from many retail vendors, which can be identified by using search terms such as "metric rule," "meter stick," or "metric stick." Printable rulers, such as the centimeter Color-square rules, can be color printed on to overhead transparency sheets to make inexpensive metric rulers.
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Coordinate measuring machines (CMMs) are used to precisely measure the dimensions of complex automotive and other product parts. NIST mechanical engineer Herb Bandy uses a CMM to measure the dimensions of a cylinder-shaped machined part.
Related LinksPrefixesSI Base UnitsBecoming Familiar with SIEveryday EstimationMetric in SportsWriting with SI (Metric) UnitsCooking ResourcesMeter Timeline (NIST)How long is a meter? (NASA - video)Using the Metric Ruler (NSTA) - PDFHow Big are Things CubeUniverscaleScale of the UniversePowers of TenOrder of Magnitude - DistanceScale of Things - nanometers - PDFMacro, Micro, Nano - Scale of Things - PDFNanoKids - Explore (available in Spanish)What is a Natometer? (IEEE)How small is small?Measurement in Sports - Height - PDFMeasurement in Sports - Distance - PDFMeasurement in Sports - Speed - PDFHidden World of Light - Meters - PDFThe Electromagnetic Spectrum (NASA) - PDFSize and Scale Lesson Plan (NNIN) - PDF