There are seven basic units of measure defined by the système international d’unités. They are called SI units and every other measuring unit is derived from them. Those seven units are:
- Temperature in Kelvin (K)
- Time in seconds (s)
- Length in meter (m)
- Mass in kilogram (kg)
- Luminous intensity in candela (cd)
- Amount of substance in mole (mol)
- Electric current in ampere (A)
Those units need to be defined in a very universal and easy way so that each and every person on the planet uses for example the same length for a meter, the same amount of time for a second, or the same mass for one kilogram. But how can you do that? For instance “The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second” as the International System of Units tells us. That puts us in need of a definition of the second, which 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.”[1] Very cryptic, but I am sure the persons who made this definition knew what they were doing.
Anyhow, if we take a look at the kilogram, we find something peculiar, namely “The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.” This tells us that the actual definition of the kilogram might change over time, since it is defined by a physical object rather than by a process or concept, which is unaffected by its surroundings. There is one kilogram prototype plus six sister kilograms which were created in 1889 of a platinum iridium alloy. They sit in an air conditioned vault close to Paris and the prototype is to the day the definition of mass. Since it is very “dangerous” for our definition to rely on a single physical object, stored in a single place, 40 other kilograms were made. They did not have the exact same weight, but the offsets were recorded. To make sure that the mass – and so our definition – does not change over time, those other kilograms were taken to locations around the globe to average potential external influences, and furthermore to be used as national kilogram standard for the different countries.
When those kilograms were brought together about 50 years later to check their masses, it was found out that despite identical storage conditions and identical material of the kilogram cylinders, their masses had changed in respect to the kilogram prototype. Not even the sister kilograms stored together with the prototype kept their masses. The process behind that is not clear and more or less a matter of speculation. That, in fact, is a big problem, because several other basic SI units depend on the kilogram, not to mention other non-basic units.
Since it is certain that something needed to be done about the problem of the changing definition of the kilogram, the Avogadro Project was brought to life. Its aim is the definition of the kilogram by the Avogadro constant (NA) using a perfect sphere of pure silicon 28. After producing said perfect sphere of single crystal, isotopically pure silicon 28, its diameter can be used for accurate calculation of the Avogadro constant, allowing the definition of the kilogram in the next step.
In the end, the question about the changing masses of platinum iridium alloy cylinders remains open. But it is great to see that one can “solve” it by changing the subject.
Andreas Neidlinger
Read more:
http://www.bipm.org/fr/si/si_brochure/general.html (last access on 12.05.2013)
http://www.ptb.de/cms/de/publikationen/zeitschriften/ptb-news/ptb-news-20103/das-neue-kilogramm-kommt-naeher.html (last access on 12.05.2013)
http://www.youtube.com/watch?v=ZMByI4s-D-Y (last access on 12.05.2013)