From very human beginnings to definitions based on gravity, the physical Earth, a standardized bar, atomic physics, and eventually the speed of light itself, we need some sort of definition of “length” that all observers can agree on in order to be useful. At last, starting in 1983, we’ve hit upon a definition of the “meter” that should finally last forever. (Credit: K. Irvine/NIST)
Until the late 20th century, there wasn’t a truly universal standard. Under our current definition, everyone agrees on what “one meter” is.
Measurement standards are needed for knowing “how much” exists.
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A teacher giving rulers to children of the second grade (8 years old) in a primary school in Vaasa, on their second day of school in Finland. The ability to measure, quantitatively, “how much” of something you have is a key aspect at the foundation of all quantitative endeavors. (Credit: Olivier Morin / Getty Images)
This Ancient Egyptian artifact shows a fragment of a cubit measuring rod. Note the markings at the bottom of the rod showing various fractions of a cubit: forerunners of divisions like inches, centimeters, and millimeters. (Credit: Gift of Dr. and Mrs. Thomas H. Foulds, 1925/Metropolitan Museum of Art)
A single “pace” was often used: around one yard/meter.
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A pace, either defined by a single stride as shown or a “return to the same foot” stride, was used as the original definition of a mile, where 1000 or 2000 paces defined that mile. Before a meter was defined by a pendulum’s length, this non-consistent standard was frequently used for similar distances. (Credit: Humphrey Muleba/public domain)
The idea of a “standard meter” came from pendulum observations.
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A pendulum, so long as the weight is all in the bob at the bottom while air resistance, temperature changes, and large angle effects can be neglected, will always have the same period when subject to the same gravitational acceleration. The fact that the same pendulum swung at different rates between different locations in Europe and the Americas was a hint toward Newton’s gravitation, and the variation of surface gravity in a latitude-dependent fashion. (Credit: Krishnavidala/Wikimedia Commons)
A swinging pendulum’s period is determined by two factors: length and gravity.
