Would you say you knew what a metre was, or a second? Odds are what you know it to mean is not the same as what it actually is.
They are all SI units that allow us to measure and describe almost any property of an object like its mass, temperature, dimensions and composition.
We use these units all the time but have you ever considered what you are really getting when you buy a kilogram of meat or how long you have actually waited when you stop for a minute?
This is an issue that has intrigued mankind since we began to trade, counting alone was not enough, and over the years we have adopted some pretty strange systems.
Business Unusual first covered this during the Rio Olympic games in 2016 when a swimming event ended with a three-way tie for second place. Swimming only measures times to two decimal places. The reason is that 50m swimming pools are not always 50m long.
The SI Units were first introduced after the French Revolution when making them more scientific was gaining popularity, not to mention the desire to get rid of anything that was considered imperial. The new system known as the metric system created new definitions for how to define the metre for distance, the cubic metre for volume and the kilogram for mass.
A metre was one ten-millionth of the distance from the north pole to the equator passing through Paris, the cubic metre was turned into the litre as being one-tenth of a metre by one-tenth of a metre by one-tenth of a metre and a kilogram was the weight of a litre of water with no impurities at four degrees Celsius.
The problem was that the original units were physical things. An actual bar one metre in length and a one-kilogram cylinder called the International Prototype Kilogram or Le Grand K. It is stored in Paris, and every other kilogram is created and adjusted to weigh the same as it does. Over the years it has been changing which is why a new option was sought.
The solution was not to use physical objects but rather mathematical constants as that would allow for very accurate calibration and so very precise measurement.
The second was updated in 1967 to use the vibrations of a caesium atom.
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.— The definition of the SI Unit for time - a second
The metre was defined by the second in 1983 as the distance light travels in a fraction of a second.
The metre is the length of the path travelled by light in vacuum during a time interval of 1 / 299 792 458th of a second.— SI Unit of for distance - the metre
But that left the kilogram, and in 1999, the General Conference on Weights and Measures decide to find a new definition. In 2005 options were being refined with the decision postponed to 2007 when it was moved to 2011. There were two options: define the number of particles that will be a kilogram or use an electromagnetic force of a determined strength to exert the equivalent of a kilogram of mass. Both are useful and as the mole is an SI Unit of substance and uses the kilogram as part of the definition both were used to determine the exact measure for the two constants they would be based on Planck's constant for mass and Avogadro's constant for the mole a unit of substance.
The current International Prototype Kilogram (IPK) also known as Le Grand K
The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J⋅s, which is equal to kg⋅m2⋅s−1, where the metre and the second are defined in terms of c and ΔνCs.— From 20 May 2019, the definition of the SI Unit for mass - the kilogram.
The vote to decide was set for 2014 but delayed once more to 2018, and on 16 November in France, the Kilogram, Mole, Ampere and Kelvin were redefined using mathematical constants none of which are physical things.
Nothing changes in the day to day lives of people who uses the kilogram although, for Liberia, Myanmar and the United States who don’t use the metric system and the UK who don’t use it for some things now might be the time to change. The units of the mentioned countries are all defined by the metric units, they just choose to convert them and use something else.
Europe tried to force the UK to change and does still require packaging to reflect at least both, but for those that grew up with the original units changing is hard. Perhaps a younger generation will be ready to switch?
Time will tell although no-one uses the correct scale for temperature which is Kelvin, not Celsius or Fahrenheit.
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