Of all the scientific standards the one that is still yet to be defined in pure scientific terms is the kilogram. Whilst all SI units, like meters, have their basis in real world objects they have since been redefined in pure scientific terms. The meter, once defined by the length of a pendulum with a half-second period, is now defined as the distance light travels in a specific time frame. The reasoning for redefining these measurements in absolute scientific terms has to do with reproducibility of standard objects as it’s almost impossible to create two objects that are exactly identical. Such is the issue that the kilogram has faced for much of its life, but soon it will change.


The picture above depicts a replica of the International Prototype Kilogram, a platinum-iridium cylinder machined to exacting specifications which defines the current day kilogram. It’s almost exactly sized brother, Le Grande K, is the standard by which all other kilogram measures are compared. There are numerous cylinders like this all around the world and they’re periodically compared to each other to ensure that they’re roughly in alignment. However over time there’s been fluctuations noted between the prime cylinder and its siblings which causes scientists all sorts of grief. Essentially since the kilogram weights are different, even by only micrograms, these variations need to be accounted for when using the kilogram as a standard. It would be far better if it was rigidly defined as then scientists would be able to verify their instruments themselves rather than having to rely on a physical object.

It seems we may have finally reached that point.

The trouble, you see, with defining something as nebulous as the kilogram in pure scientific terms is that it needs to be reproducible and verifiable. The International Committee for Weights and Measures (CIPM) agreed to express the kilogram in terms of Planck’s constant (a link between a photon’s energy and its frequency). Essentially experiments would need to be designed to calculate the Planck value using the standard kilogram weight as a measure, which would then allow scientists to describe the kilogram as a function of a physical constant. There were numerous experiments designed to test this however the two that have come out on top were: creating a single crystal silicon sphere and counting the atoms in it and using a device called a watt balance to measure the standard kilogram against an electrical force. These are both scientifically sound ways of approaching the experiment however the latter method struggled to get the required results.

Essentially, whilst the experiment was capable of producing usable results, they couldn’t get the level of tolerances that would be required for verification of Planck’s Constant. It took several rounds of experiments, and several different research teams, to close in on the issues however in August this year they managed to hone in on Planck’s Constant with an uncertainty of 12 parts per billion, enough for the CIPM to accept the results for use in verifying a standard kilogram. This means that these results will likely now for the basis for scientists the world over to validate and calibrate devices that reference the kilogram without having to get their hands on one of the platinum-iridium cylinders.

The change of definition isn’t slated to come into effect until July 2017 and there’s further experimentation to be done between now and then. There is potential for one of the experiments to cause an upset with the other as any deviations from the currently accepted results will require confirmation from both. Currently the silicon sphere experimenters are in the process of procuring some additional test items for investigation which could potential cause this whole thing to start over again. However with the watt-balance experiment now having most of the major kinks worked out it’s unlikely this will occur and the further experimentation will ensure that the error rate is reduced even further.

It won’t mean much of a change to our everyday lives, we’ll continue weighing things with the same scales as we did before, but it will mean a monumental change in the way we conduct scientific research. Finally ridding ourselves of the last physical objects that define our measurements will free us from their variability, making them accurate in the most true sense. It’s been a long time coming but there’s light at the end of the tunnel and we’ll soon have no need for those platinum-iridium cylinders. Well, not unless you fancy yourself a really expensive paperweight.

About the Author

David Klemke

David is an avid gamer and technology enthusiast in Australia. He got his first taste for both of those passions when his father, a radio engineer from the University of Melbourne, gave him an old DOS box to play games on.

View All Articles