The New SI

Did you ever wonder how we got the International System of Units in the first place? Why is it changing now? Read more to explore its history and learn what is changing.

In the beginning…

The earliest measurement systems were derived from the body parts of the rulers of the region. As civilizations became more mobile and trade started occurring between people from different regions the need for standardization arose.

Cheating and/or feeling cheated were common aspects of trade, so during the French Revolution King Louis XVI of France proposed a standardized system of measurement to crack down on the swindlers.

In 1790, a panel of five leading French scientists was appointed to investigate weights and measures. The panel decided that the new measure of length should be equal to one ten-millionth of the distance from the North Pole to the Equator (the quadrant of the Earth’s circumference), measured along the meridian passing through Paris and that the unit of weight should be that of a cube of water whose dimension was a decimal fraction of the unit of length. Using the science of the time, provisional values were assigned to the meter and the kilogram.

In 1795 the metric system was formally defined as the law of France and in 1799 reference copies of the meter and kilogram were manufactured in platinum and remained the standards of measure for the next 90 years.

The Birth of the International Standard of Units

This was all well and good for France, but by the 1800s globalization was occurring, so seventeen nations got together to establish an international system. They conceived a system of measurement that was derivable from unchanging phenomena, but due to the scientific limitations of the time they all agreed to use the kilogram that had been established in France. On May 20, 1875 the Treaty of the Meter was signed to establish international prototypes for mass and length. New reference copies were made and distributed to member countries.

Three international organizations were set up to take custody of the international prototype kilogram and meter and to regulate comparisons with national prototypes. One of those organizations, The General Conference of Weights and Measures, published the International Standard of Units (SI) in 1960. The SI was built on six base units: ampere, kelvin, second, meter, kilogram, and candela. With the publication of the SI the meter was redefined in terms of the wavelength of light from a specified source, making it derivable from natural phenomena, leaving the prototype kilogram as the only artifact upon which the SI unit depended on.

The New SI

Since 1960, technological advances have made it possible to address various weaknesses in SI. Several of the definitions of the base units have been refined. But the biggest fault was the kilogram’s definition still being based on a physical artifact.

During the third periodic recalibration between the International Prototype Kilogram (IPK) and the various national prototypes in 1988-89 the average difference was 50 μg. Had the IPK been losing mass or the national prototypes been gaining mass? There was no way to tell, so in 2007 the General Conference on Weights and Measures mandated the investigation of using natural constants as the basis for all units of measure rather than using artifacts.

In 2010 the Consultative Committee for Units presented to the International Committee for Weights and Measures, one of the other organizations established with the signing of the Treaty of the Meter, the following resolution and draft changes to the SI brochure:

  • In addition to the speed of light, four constants of nature—Planck’s constant, an elementary charge, Boltzmann constant and Avogadro’s number—be defined to have exact values;
  • The international prototype kilogram be retired;
  • The current definitions of the kilogram, ampere, kelvin and mole be revised;
  • The wording of the definitions of all the base units be tightened up.

They set to work to on the new definitions, but at the 2014 meeting of the General Conference on Weights and Measures it was found that despite the progress in the necessary requirements the data did not yet appear to be sufficiently robust for them to adopt the revised SI thus postponing the revision to the next meeting, this year 2018.

The international metrology community has been hard at work on getting the uncertainties in the neighborhood of 20 parts per billion and making sure the measurements can be made reliably and repeatably over time in order to satisfy the requirements of the General Conference of Weights and Measures. It is believed that they have satisfied the requirements, so the new SI is expected to be accepted this November.

Watch this interesting video for the science behind the new definitions.

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