Today In Science History – June 22

June 22 is an important day in the history of the standards of measurement used in science today. On June 22, 1799 the first official standard meter and kilogram were presented at the Archives de la République in Paris.

King Louis XVI charged leading scientists of the time to create a system of measurements that would lead to the creation of the metric system. The base measurements would all be based on reproducible physical standards. Other measurements would all be derived from the base units by powers of 10.

The meter was to be the standard of length. Two ideas were put forth for this standard. One was the length of a pendulum necessary to have a period of one half second. The other was one ten-millionth of the length of the Earth’s meridian along a quadrant, or one fourth the circumference of the Earth from North to South. Ultimately, the meridian length was chosen since the force of gravity varies from place to place which would alter the pendulum’s period. The official length of one meter would be 10-7 of the distance from equator to pole along the meridian passing through Paris.

The kilogram was originally supposed to be called a “grave” and set to be equal to the mass of one liter of water at the ice point (very close to today’s 1 kilogram), The French Revolution interrupted this standardization work and when the new government got involved, they preferred a standard unit called a “gramme” based on one cubic centimeter of water at the ice point. The gramme standard had one problem. It was too small amount of mass for trade. Typically, goods do not change hands on the gram scale of measurements. A larger unit of mass was needed. A compromise was reached where the gramme would still be used, but the official physical standard would be the size of the original “grave”. Since the gramme was 1/1000th the size of the gramme, it took 1,000 grammes to equal one of the new standard kilogramme. This is why the kilogram the only Metric standard to contain a prefix in its name.

The standards for meter and kilometer have changed over the years. A meter is now the distance light travels in a vacuum in 1/299792458 of a second. The kilogram is represented by a mass of platinum-iridium alloy kept at the Bureau International des Poids et Mesures (BIPM) in Paris.

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