The definition of meter has changed dramatically several times. The last major change was exactly 40 years ago. How was the length of a meter determined?
The origins of the abacus go back to the French Revolution. During that period, people wanted many features from it Old system They were replaced, including traditional units of measurement (Charlemagne’s foot was often used as a measure of length).
Scientists had previously proposed using a second pendulum. This was like a weight on a rope that took exactly two seconds to swing back and forth. The required length of this rope was then used as a standard length measure. But this proposal was eventually scrapped because gravity is slightly different everywhere on Earth, and so is the length of the rope.
Liter and kilo
In 1970, the French Academy of Sciences established a council of leading scientists and mathematicians tasked with coming up with a better alternative. A year later he made a recommendation. The proposed measure of length, the meter (derived from the Greek word metron, meaning “measure”), was a ten-thousandth of the distance between the North Pole and the equator. The line drawn for this purpose – of course – ran through Paris.
They also immediately came up with the measurement of volume and the unit of mass. A liter is the volume of a cube of distilled water whose dimensions are one-thousandth of a cubic metre. The kilogram in turn became the weight of a liter of distilled water in a vacuum.
In 1792, astronomers Pierre Mischin and Jean-Baptiste Delambre measured the distance between Dunkirk, France, and Barcelona, Spain, and then calculated the distance between the North Pole and the Equator. After about seven years they reached their final size. The Academy had a platinum bar made of exactly this length – the prototype of the meter.
It was later discovered that scientists had made errors in calculating the curvature of the Earth, causing the model scale to be 0.2 mm shorter than the actual agreed-upon length. However, it was decided that the counter would remain as recorded on the original platinum bar.
More and more countries adopted the metric system, which required more prototypes. But due to frequent use, the bars became worn out. Therefore, they were at one time made of platinum and iridium, which was much stronger than platinum alone.
The bars were no longer flat either, but had an X-shaped cross-section to better resist deformations. Furthermore, the new prototypes were no longer the final standard, as the scale was determined by the ends of the rod. Instead, the bars were longer and a meter was defined as the distance between two lines on a surface. These engravings were easier to make than a final standard, and they ensured that the length of the scale remained unchanged if the ends of the tape were damaged.
The speed of light
Many years have passed and the definition has remained unchanged. Until scientific progress made it possible to determine the length of a meter with a physical constant that can be measured everywhere on Earth. When electrons from the krypton-86 atom make an energy jump, they release energy in the form of red-orange light that always has the same wavelength. Exactly 1,650,753.73 of those wavelengths are the same as the meter length recorded in platinum and iridium rods.
But this definition did not last long, and on October 21, 1983, the meter was given a new, more reliable definition. Scientists have succeeded in measuring the speed of light with great accuracy. Since light has an incredibly high but ultimately finite speed, distances can be calculated using the simple formula: distance = speed x time. This led to the new definition: a meter is the distance light travels in 1/299,792,458 seconds in a vacuum.
meters in the yard
This definition still applies. In 2019, a small addition was made that also defines the second with physical constants. Each definition of scale was more reliable than the last, but length remained the same throughout.
Today, most countries use the metric system. But countries that use other length measurements (such as the United States) define it using a scale. For example, the yard has been officially defined as exactly 0.9144 meters since 1959.
sources: Nest, Britannica
Photo: Patricia Serna via Unsplash
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