One Sunday afternoon, Mrs. Kim went grocery shopping with her family. As the temperature was below 2℃, they brought winter jackets with them and got inside the car. The GPS indicated that it would take about 20 minutes to drive to the supermarket that was about 5 km [3.1 mi] away.
First of all, she went to buy shoes for her husband. He tried on shoes (275 ㎜ and 280 ㎜) and chose ones in the larger size. In the grocery store, they bought beef (600 g) for dinner, and two bottles of milk (1 L) on discount, and a box of apples (5 ㎏) as well.
Temperature, distance, weight, and all different sizes … We are surrounded by countless numbers all our lives. How were these units of measure made?
Units, the Barometer of Civilization
There was a growing need for units as humans started farming and civilization sprouted. The development of weights and measures was necessary in the process of measuring farming areas and distributing harvested crops. They had to know the volume or weight of an object for a fair trade and accurately measure materials to make products like clothes and to construct buildings.
The earliest units of measurement were based on human body parts. In ancient Greece, the “pous” was the term for the length of a foot and the “daktylos” for the width of a finger; a pous was divided into 16 daktylos or digits. These units of measure based on human body parts or the abilities of humans are called “身體尺 (shintaishaku)” in Japan. The shaku (尺) is the distance from the tip of the thumb to the tip of the forefinger when extended, and the cubit from Egypt is based on the distance from the tip of the middle finger to the bottom of the elbow. In the Western world, the digit was the smallest basic unit, being the width of a finger.
However, it is hard to unify body-based units of measure because everyone has a different body size. In order to resolve this problem, Egypt established a standard cubit, called the Royal Cubit, which was based on the length of the Pharaoh’s forearm, and used it for building the pyramids.
Grains were also used as a unit of measurement because they’re solid and easy to get. In the third century B.C., the Qin dynasty used the length of a bamboo pipe instrument called the huangzhong as a standard unit of measurement; each had the length of lining up 90 grains of millet. They used millets as a unit of measure. A smaller unit “chun (寸)” was the length of lining up 10 grains of millet.
Grains were used to measure weights, too. The Romans decided 1,728 carob seeds to weigh one Roman pound and 144 carob seeds to weigh one Roman ounce. The term “carat” is derived from the carob seed, the original unit of measure for diamond weight. In British Imperial units, one pound was equal to 7,000 grains of barley. However, grains were not appropriate to be used as a standard unit of measurement because their weights and lengths varied according to weather conditions and their sizes differed from one another.
Efforts for Unification of Units
On July 23, 1983, an Air Canada Boeing 767 flying to Edmonton ran out of fuel and the warning system sounded. The engines stopped, and the pilot barely made an emergency landing. This accident was caused by confusion over units of measure. The pilots calculated the amount of needed fuel in kilograms, but ground staffs fueled the aircraft in pounds. Consequently, the airplane was flying with less than half the required fuel.1
1. 1 L = 0.8 ㎏ [1.77 lb]
In September 1999, the NASA’s Mars Climate Orbiter was lost when it entered the Martian atmosphere. This was also due to confusion over units of measure. NASA uses the metric system of meters and kilograms, but the company that built the spaceship used the Imperial system of units based on yards and gallons. The unit confusion was not noticed even when the spacecraft was entering Mars’ atmosphere, which caused the accident.
Throughout all ages, the disunified system of units has caused confusion. In France, there were over 250,000 different units of measurement in use during the 18th century. This caused communication and trade problems, and the lords could collect taxes at their own discretion. These kinds of problems happened in the East as well as in the West. The chi (尺) was originally based upon the distance measured by a human hand from the tip of the thumb to the tip of the forefinger, but its standard length became longer over time. When the Government of Meiji Japan adopted it in 1875, its length was 30 centimeters. This also happened as governors continued to increase its length to collect more taxes.
Scientists had long dreamed of developing scientific units that are consistent and easy to use for everyone. Christopher Wren, a 17th century English architect and astronomer, tried to determine the standard unit of length by means of the oscillation of the pendulum. He proposed a new system based on the yard, which he defined as the length of a pendulum swing beating at the rate of one per second. Various means such as Capillary action (or capillarity2) and light wavelengths were discussed, but an agreement was finally made on the determination of the earth’s circumference as a standard unit of measure.
2. A phenomenon where ascension of liquids through a narrow tube or cylinder takes place; liquid level in the capillary is above or below the liquid level outside the capillary.
That’s how the metric system, a system of units for measurement, was developed. The metre or meter, derived from the Greek word “metron” meaning “measure” was originally defined as one ten-millionth of the distance from the equator to the North Pole, measured through Paris, France. The French astronomers Delambre and Méchain undertook an expedition to measure the distance. On December 10, 1799, the metric system came into force in France, and the metre was established as the standard measure across the country. They also made one-metre long rulers and distributed them to each city.
Napoleon, who conquered most of Europe, spread the metric system during the 19th century. The metric system began to be used in a wider range of fields as the Metre Convention was signed in France on May 20, 1875, by representatives of 17 nations, including the United States, Germany, Russia, and Brazil.
Modification of Units
Strictly speaking, the Earth is not a perfect sphere because it has an equatorial bulge. So, the Earth has a different diameter when measured around the equator than it does when measured from the poles. This means that the length of the metre, which is based on the circumference of the Earth, could change. At the first General Conference on Weights and Measures [CGPM] in 1889, scientists defined the length of a metre based on a cylinder of platinum and iridium with a high chemical stability.
However, an artifact changes slightly according to the surrounding environments such as the temperature and humidity; it can expand or contract or oxidize. The metric system is not an exception. After multiple researches, scientists agreed to a new definition of the metre using atoms in 1960.3
3. Equal to 1,650,763.73 wavelengths of the orange-red emission line in the spectrum of the krypton-86 atom in a vacuum.
They did not stop here, and in 1983 they defined the metre with reference to the speed of light that doesn’t change. The definition states that the meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second. The “second” here is based on the oscillations of the caesium 133 atom that doesn’t change.4 Ironically, it is still impossible to determine the length of a metre that satisfies the new definition because perfect vacuum cannot be achieved artificially. A helium-neon gas laser is used to measure the length of a meter bar in scientific circles.
4. A second is the duration of 9,192,631,770 oscillations of a caesium 133 atom.
There are seven basic units in the International System of Units [SI]: the meter [m] for measurement of length, the kilogram [㎏] for mass, the second [s] for time, the ampere [A] for electric current, the kelvin [K] for temperature, the mole [mol] for amount of substance, and the candela [cd] for luminous intensity. The definitions of these units have been constantly modified and changed to become unchanging standards, keeping up with advances in science and technology.
The kilogram was the last of the SI units to be defined by a physical prototype, leaving it the only artefact upon which the Si Unit definitions depend. The International Prototype of the Kilogram [IPK] is a cylinder made from an alloy of 90% platinum and 10% iridium, with a diameter and height of 39 ㎜. This cylinder sits under three protective glass bells, in a temperature-and humidity-controlled environment, being carefully checked by the International Bureau of Weights and Measures near Paris every year.
However, the International Prototype of the Kilogram [IPK] couldn’t avoid being redefined with the passing of time. There was in increasing gap in mass between the IPK and its copes stored in 23 countries. In 2007, the mass difference between the IPK and its copies was 100 µg5.
5. One microgram [㎍] is 1 millionth of a gram.
The General Conference on Weights and Measures agreed to redefine the kilogram in terms of the Planck constant [h]6, a fundamental physical constant. The kilogram, the SI unit of mass, is defined by taking the fixed numerical value of the Planck constant [h] to be 6.62607015×10−34 when expressed in the unit joule second [J·s], which is equal to ㎏·㎡·s-1.
6. The quantum of electromagnetic action that relates a photon’s energy to its frequency.
After several years of research using a Kibble balance—a convergence of cutting-edge technology, the value of the Planck constant was fixed7. Through this, the kilogram was redefined at the 26th General Conference on Weights and Measures held in Versailles, France, on November 16, 2018. This took place about 130 years after the International Prototype of the Kilogram was made in 1889. As of May 20, 2019, World Metrology Day, the new definition of the kilogram has been adopted.
7. The Planck constant [h] = 6.62607015×10-34 J·s [㎏·㎡/s]
The units that never change regardless of time and space, anywhere in space! There is still an ongoing desire among people to set up unchanging standards, through various attempts and researches. One of the impressive things is that the basic standards of measurement have changed from artifacts to natural objects according to advances in science. Time, known as the “standards of standards,” is defined based on light.
Eternal, unchanging standards cannot be easily determined or established by human abilities. Only the laws of nature, which God has given to all people in creation, are the unchanging and eternal standards.
“Where were you when I laid the earth’s foundation? Tell me, if you understand. Who marked off its dimensions? Surely you know! Who stretched a measuring line across it?” Job 38:4–5
“Can you fathom the mysteries of God? Can you probe the limits of the Almighty? They are higher than the heavens–what can you do? They are deeper than the depths of the grave–what can you know? Their measure is longer than the earth and wider than the sea.” Job 11:7–9
- Reference
- Crease, Robert P. World in the Balance: The Historic Quest for an Absolute System of Measurement. New York, NY: W. W. Norton & Company, 2012.
- Tadahiko, Hoshida. Stargazing Unit Story (Korean Edition). Korea: About Book, 2016.
- Kim, Il-seon. 단위로 읽는 세상 [Read the World Through Units]. Korea: Gimm-Young Publishers, 2017.