How do we define everything around us? By it's shape, colour, or texture? Defining a chair as 'medium sized' and 'brown' may be useful in the subjective art of interior decoration, but to an objective eye they're not very helpful. Imagine then if we are to describe the motion of planets, the orbit of satellites, the flow of water through a hydroelectric dam, or the flow of electricity in a microprocessor. Subjectivity is not an option, and any impreciseness or subjectivity in standards may spell disaster.
It's a good thing we figured that out in the 19th century, as without it it would not make the 20th and 21st centuries possible. |
"In metric, one milliliter of water occupies one cubic centimeter, weighs one gram, and requires one calorie of energy to heat up by one degree centigrade—which is 1 percent of the difference between its freezing point and its boiling point. An amount of hydrogen weighing the same amount has exactly one mole of atoms in it."
Josh Bazell, Wild Thing
In 1668 John Wilkins, an English clergyman, proposed a coordinated system of units of measure for length, area, volume, and mass for use by philosophers. His unit of length, the “standard”, was the length of a pendulum that had a half-beat of one second, the units of area and volume were the square and cubic standard respectively and the unit of mass was the mass of a cubic standard of rainwater. Two years later Gabriel Mouton, the vicar of St. Paul’s Church in Lyons, France, proposed a unit of length based on one minute of arc of a great circle of the earth (now called a nautical mile, 1852 m). He also proposed a coordinated way of naming the decimal multiples and submultiples of the basic unit of length. However the time was not right, either politically or technologically for the introduction of systems proposed by either Wilkins or Mouton.
By the outbreak of the French Revolution, both the political climate and technological advances made reform of the system of units of measurement desirable. The sponsor of weights and measures reform in the French Revolutionary National Assembly was the Bishop of Autun, better known as Talleyrand. Under his auspices, the French Academy appointed several committees to carry out the work of developing a usable system of weights and measures for France. One of the committees recommended a decimalized measurement system based upon a length equal to one ten-millionth of the length of a quadrant of the earth’s meridian (i.e., one ten-millionth of the distance between the equator and the North Pole).
In 1790, in the midst of the French Revolution, the National Assembly of France requested the French Academy of Sciences to deduce an invariable standard for all the measures and all the weights. The Commission appointed by the Academy created a system that was, at once, simple and scientific. The unit of length was to be a portion of the Earth’s circumference. Measures for capacity (volume) and mass were to be derived from the unit of length, thus relating the basic units of the system to each other and to nature. Furthermore, larger and smaller multiples of each unit were to be created by multiplying or dividing the basic units by 10 and its powers. This feature provided a great convenience to users of the system, by eliminating the need for such calculations as dividing by 16 (to convert ounces to pounds) or by 12 (to convert inches to feet). |
Similar calculations in the metric system could be performed simply by shifting the decimal point. Thus, the metric system is a decimal (base 10) system.
The Commission assigned the name “metre” (spelled with the non-standard spelling “meter” in the US) to the unit of length. This name was derived from the Greek word, metron, meaning “a measure”. The physical standard representing the metre was to be constructed so that it would equal one ten-millionth of the distance from the North Pole to the equator along the meridian running near Dunkirk in France and Barcelona in Spain.
A surveying team under the direction of two men, Pierre-Francois-Andre Mechain and Jean- Baptiste-Joseph Delambre, spent 6 years in measuring the “arc” that the earth made in a line between Dunkirk in France on the English Channel and Barcelona in Spain. The surveyors underwent much harassment and even were jailed, at times, while making their measurements, because some of the citizens and area officials resented their presence and felt they were up to no good. It was later found that Delambre and Mechain had not properly accounted for the earth’s flattening in correcting for oblateness. However, the metre remains the invariable standard for the metric system, and its length has not changed even though the official expression of the definition the metre has changed several times to improve the accuracy of its measurement.
Meanwhile, scientists were given the task of determining the other units, all of which had to be based upon the metre.
The Commission assigned the name “metre” (spelled with the non-standard spelling “meter” in the US) to the unit of length. This name was derived from the Greek word, metron, meaning “a measure”. The physical standard representing the metre was to be constructed so that it would equal one ten-millionth of the distance from the North Pole to the equator along the meridian running near Dunkirk in France and Barcelona in Spain.
A surveying team under the direction of two men, Pierre-Francois-Andre Mechain and Jean- Baptiste-Joseph Delambre, spent 6 years in measuring the “arc” that the earth made in a line between Dunkirk in France on the English Channel and Barcelona in Spain. The surveyors underwent much harassment and even were jailed, at times, while making their measurements, because some of the citizens and area officials resented their presence and felt they were up to no good. It was later found that Delambre and Mechain had not properly accounted for the earth’s flattening in correcting for oblateness. However, the metre remains the invariable standard for the metric system, and its length has not changed even though the official expression of the definition the metre has changed several times to improve the accuracy of its measurement.
Meanwhile, scientists were given the task of determining the other units, all of which had to be based upon the metre.
The initial metric unit of mass, the “gram”, was defined as the mass of one cubic centimetre — a cube that is 0.01 metre on each side — of water at its temperature of maximum density. For capacity, the “litre” (spelled “liter” in the US) was defined as the volume of a cubic decimetre — a cube 0.1 metre on each side.
After the units were determined, the metric system underwent many periods of favor and disfavor in France. Napoleon once banned its use. However, the metric system was officially adopted by the French government on 7 April 1795. A scientific conference was held from 1798 to 1799 (with representatives from the Netherlands, Switzerland, Denmark, Spain, and Italy) to validate the metric system’s foundation and to design prototype standards. Permanent standards for the metre and the kilogram were made from platinum. These standards became official in France by an act of 10 December 1799. Although the metric system was not accepted with enthusiasm at first, adoption by other nations occurred steadily after France made its use compulsory in 1840. The standardized structure and decimal features of the metric system made it well suited for scientific and engineering work. Consequently, it is not surprising that the rapid spread of the system coincided with an age of rapid technological development. In the United States, by Act of Congress in 1866, it became lawful throughout the United States of America to employ the weights and measures of the metric system in all contracts, dealings or court proceedings. |
The Great 2019 Metric Makeover
After decades of groundbreaking laboratory work, the world’s scientific and technical community has redefined four of the seven base units for the International System of Units (SI). A vote to adopt the change happened on November 16, 2018, at Versailles, France, and the change went into effect on May 20, 2019. The units in the revised SI are based completely on seven unchanging quantities or “universal constants,” including the speed of light, the amount of electric charge in an electron, and the Planck constant. Learn more about each of these “invariants of nature” and how they come into play in the revised SI. c, the speed of light in vacuum: The speed limit in the universe is the speed of light in vacuum (empty space). Nothing in the cosmos—matter, information, energy—can travel faster than c. c is equal to 299,792,458 meters per second. In the revised SI, c helps to define the meter, kilogram and kelvin. h, the Planck constant: The Planck constant is one of the fundamental constants of quantum mechanics. Quantum mechanics shows us that energy is exchanged and absorbed in specific amounts, known as “quanta.” h defines the size of those quanta, which can be visualized as packets of energy exchanged by matter. |
In the revised SI, the Planck constant h is equal to exactly 6.626 070 15 × 10-34 Joule seconds. This value for the Planck constant was only finalized in 2017, and that value is key to redefining the kilogram. h helps to define the kilogram, kelvin and candela.
e, elementary charge: e is the amount of charge in an electron. It’s connected to electromagnetism, one of the four forces of nature. In the revised SI, e is equal to 1.602176634 × 10-19 coulombs. It helps to define the ampere.
∆νCs, the hyperfine transition frequency of cesium-133: This might sound (and look) like sci-fi jargon, so let’s break this down a little bit. Cesium is a metal atom in the periodic table. Cesium-133 is its most common form, or isotope, containing a total of 133 protons and neutrons. Like all atoms, cesium is orbited by electrons. The energy of cesium’s outermost electron can be controlled with microwave radiation. The frequency of microwave radiation that causes this electron to jump between two closely spaced low-energy states is known as the hyperfine transition frequency. The hyperfine transition frequency ∆νCs is equal to 9,192,631,770 hertz. In the revised SI, ∆νCs helps to define the second, meter, kilogram and ampere. k, the Boltzmann constant: The Boltzmann constant relates an object’s energy to its temperature. In the revised SI, the Boltzmann constant k equals 1.380649 × 10-23 joules/kelvin. k will help to define the kelvin. |
NA, the Avogadro constant: The Avogadro constant defines the number of particles in a mole, the SI unit that expresses the amount of substance. Simply put, Avogadro’s number of electrons equals one mole of electrons. Similarly, Avogadro’s number of water molecules equals one mole of water.
In the revised SI, NA equals 6.02214076 x 1023 particles per mole.
Kcd, the luminous efficacy of monochromatic radiation of frequency 540 × 1012 hertz: This last constant is arguably one of the most artificial and human-centered. To explain it, let’s decode the entire second half of the definition first. “Monochromatic radiation of frequency 540 × 1012 hertz” is simply green light, specifically the shade of green light that the human eye can pick up most sensitively in a well-lit room.
And “luminous efficacy” is basically the total amount of visible light that a source produces using a certain amount of power. Kcd is equal to 683 lumens per watt and helps to define the candela.
In the revised SI, NA equals 6.02214076 x 1023 particles per mole.
Kcd, the luminous efficacy of monochromatic radiation of frequency 540 × 1012 hertz: This last constant is arguably one of the most artificial and human-centered. To explain it, let’s decode the entire second half of the definition first. “Monochromatic radiation of frequency 540 × 1012 hertz” is simply green light, specifically the shade of green light that the human eye can pick up most sensitively in a well-lit room.
And “luminous efficacy” is basically the total amount of visible light that a source produces using a certain amount of power. Kcd is equal to 683 lumens per watt and helps to define the candela.
Ponder this
The French men who defined the meter, also indirectly defined the standards of weight, temperature, electric current, etc. But not the second, the basic measurement of time. Why didn't they?
Why was the kilogram the last to be defined through the use of a natural, universal constant?
Discuss
The current measurement standards were derived from arbitrary definitions that was semi-scientifically conceived back in the 18th century. If we were to redefine such standards, how should we do it? How much time would a second be? How long is a standard length? How heavy is a standard weight? If the scientists of the 18th century were to have our knowledge and technology, how would they define the standard measurements?
Further reading
International System of Units, is the modern form of the metric system.
Bureau International des Poids et Mesures, an intergovernmental organisation through which member states act together on matters related to measurement science and standards.
Fandel, Jennifer. The Metric System. Mankata, MN: Creative Education, 2006.
Hebra, A. Measure for Measure: the Story of Imperial, Metric, and Other Units. Baltimore: Johns Hopkins University Press, 2003.