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. The report into the incident, quaintly called a “mishap” (which cost $193.1 million in 1999), said:
[…] the root cause for the loss of the MCO spacecraft was the failure to use metric units in the coding of a ground software file, “Small Forces”, used in trajectory models.
Essentially the spacecraft was lost in the atmosphere of Mars as it entered orbit lower than planned.
The new SI definitions
So why the change today? The main problems with the previous definitions were, in the case of the kilogram, they were not stable and, for the unit of electric current, the ampere, could not be realised.
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 today. (All the technical details are
available here.)
Australia had a hand in fashioning the roundest macroscopic object on the Earth, a silicon sphere used to measure the
Avogadro constant, the number of entities in a fixed amount of substance. This now defines the SI unit, mole, used largely in chemistry.
From standard to artefact
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.
From today 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.
[…] 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 metre and the second are defined in terms of c and ΔνCs.
Try explaining that to someone!