How much is a pound? 1,000 grams. 2,20462 pounds. Or 0.0685 slugs based on the old imperial gravity system. But where does it actually come from and how can everyone be sure they use the same measurement?
Since 1889, the countries that are members of the General Conference on Measures and Weights have agreed to use a standard block of metal products near Paris to determine the kilogram. But although the modern block is stored in an extremely controlled environment, its weight can be changed in tiny quantities as its wear causes mass and dirt loss causing it to increase. To address this problem, scientists around the world have spent nearly two decades discussing how kilo could be determined in relation to steady nature measurements. And now they have finally decided.
The first kilo (originally named tomb) was designated in 1793 by a committee of the French Academy of Sciences, who wanted a better standard than the constant quantities of grain traditionally used. The committee decided that the new measure would be the mass of one cubic centimeter of distilled water at 4 ° C (the temperature at which water has the highest density under normal conditions). This had the advantage that the most appropriately equipped laboratories could reproduce this pattern. Then a prototype of this mass was turned into brass.
Unfortunately, this mass definition was dependent on another variable, the meter. At this point, the meter was temporarily set only as part of the distance from the North Pole to the Equator. Once the meter value and the water temperature at its most densely determined, the kilo should also be replaced. And a new prototype was molded into platinum to represent this mass.
Eventually, this was replaced with the IKP currently used, is thrown by a mixture of platinum and iridium to make it very hard and to prevent its reaction with oxygen. The IPK and six copies are kept by the International Bases and Measures Office at Pavillon de Breteuil, Saint-Cloud, near Paris in France, to act as a reference for action against. Copies of IPK are shipped around the world to ensure that all participating countries use the same standard.
But even modern IPK can gradually change into mass. In fact, the International Bureau and Measure Office's response is to revise the definition of a kilogram, as well as all other basic units of measurement used in science (known as SI units, from French to the international system).
Instead of measuring the kilogram in a block stored in a treasure, we can define it based on exact values of nature constants. The agreement on a definition has lasted a long time because we had to be able to measure these constants to strict standards with an uncertainty of 30 parts per billion (ie the measurements are accurate at 0.00000003 of a unit).
Scientists have already done this for time and length. One second is no longer a fraction of the time Earth needs to rotate, which can change as the ball accelerates or slows down. Instead, a second is now defined by the time it takes to release a certain amount of energy as irradiation from Cesium-133 atoms. In particular, one second equals 9.192.631.770 transitions to the Cesium-133 terrain high state. This is the same regardless of when or where it is measured.
Scientists were able to redefine the meter in relation to the second and other physical constant, the speed of light in a gap (c), which scientists have calculated as 299,792,458 meters per second. So a measure is now the length of the light travels in 1 / c seconds.
The new definition of kilo uses a measurement from another fixed value by nature, the Planck constant (h), which will be set to 6.62607015 × 10-34 joule seconds. Planck's constant can be found by dividing the amount of energy a particle of light or "photon" carries from its electromagnetic frequency.
The constant is usually measured in joule-seconds, but it can also be expressed in pounds square meters per second. We know what is a second and a measure of the other definitions. So adding these metrics together with an accurate knowledge of Planck's stability, we can get a new, very precise definition of kilo.
Part of the reason that created the new definition has been so long is that scientists had to create very accurate devices to measure Planck's constant with a fairly high degree of precision. The method has also been questioned because it will break the relationship of the kilogram to other basic SI units, especially the mole, which measures the amount of a substance in terms of the number of particles it originates from. Some scientists have suggested alternative methods as a result.
But after a symbolic vote, the new definition of kilo will be used by the International Bureau of Standards and Measures and by the national measurement institutes around the world, along with new definitions of the other SI units, the mole, kelvin (temperature), the amber (current) and candela (luminous intensity).
For most people, day-to-day life will normally continue despite redefining. A standard bag of sugar will contain as much sugar as ever. But some of these changes, for example in kelvin, will mean practical advantages for scientists who make very accurate measurements. And to answer the question "how much is a kilo," we no longer have to compare platinum blocks or worry about scratching them.
Kevin Pimbblet is Senior Lecturer in Physics at Hull University. This article was originally shown in The Chat.