The British, who had been prominent in the discussions and had provided the platinum-iridium kilogram, refused to sign the treaty until 1884.
Even then the new system was only used by scientists, with everyday life being measured in traditional Imperial units such as pounds and ounces, feet and inches.
The United States signed the treaty on the day, but then never actually implemented it, hanging on to its own version of the British Imperial system, which it still mostly uses today.
The US may have rued that decision in 1999, however, when the Mars Climate Orbiter ( MCO ) went missing in action. Thereport into the incident , quaintly called a 'mishap,' which cost US$193.1 million in 1999, said:
Essentially the spacecraft was lost in the atmosphere of Mars as it entered orbit lower than planned.
Lost on Mars: An investigation found the Mars Climate Orbiter likely burned up in the atmosphere of the red planet due to a clash of metrics. Image: NASA/JPL
The new SI definitions
So why the change today? The main problems with the previous definitions were, in the case of thekilogram , they were not stable and, for the unit of electric current, the ampere, could not be realized.
And from weighings against official copies, we think the Big K was slowly losing mass.
All the units are now defined in a common way using what the BIPM calls the ' explicit constant ' formulation.
The idea is that we take a universal constant – for example, the speed of light in a vacuum – and from now on fix its numerical value at our best-measured value, without uncertainty.
Reality is fixed, the number is fixed, and so the units are now defined.
We therefore needed to find seven constants and make sure all measurements are consistent, within measurement uncertainty, and then start the countdown to Monday. (All the technical details areavailable here .)
Australia had a hand in fashioning the roundest macroscopic object on the Earth, a silicon sphere used to measure theAvogadro constant , the number of entities in a fixed amount of substance. This now defines the SI unit, mole, used largely in chemistry.
Walter Giardini of the National Measurement Institute Australia holding a silicon sphere as part of the Avogadro project. Photo: Brynn Hibbert
From standard to artifact
What of the Big K – the standard kilogram? Today it becomes an object of great historical significance that can be weighed and its mass will have measurement uncertainty.
Now the kilogram is defined using the Planck constant, something that doesn't change from quantum physics.
The challenge now though is to explain these new definitions to people – especially non-scientists – so they understand. Comparing a kilogram to a metal block is easy.
Technically a kilogram (kg) isnow defined :
[…] by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the meter and the second are defined in terms of c and ΔνCs.
Try explaining that to someone!
Update: the phrase 'tens of centimeters' was changed to 'tens of millimeters' at the request of the author.
David Brynn Hibbert , Emeritus Professor of Analytical Chemistry,UNSW
This article was republished fromThe Conversationunder a Creative Commons license. Read theoriginal article .
<>