# NPL Realisation of SI Unit Definitions

I though that I should make it clearer exactly which of the SI units will be changed.  There are 7 fundamental SI units that all others are derived from.  This will probably be the biggest ever single change to our fundamental units since they started.  Great effort has been made to remove historical artifacts from our definitions because any variation in these artifacts change our standard to which everything else is measured against.  Its also very difficult to scale up or down from an artifact without further uncertainties creeping in.  It also means that as we get better and more accurate at measuring everything, the fundamental constants keep changing as they get more and more decimal places at the end.  If we fix the fundamental constants they will e forever constant and the unit definition can change getting more accurate with our increasing technology.  However, there are growing suspicions that our fundamental constants may not be absolutely constant after all, but this is not the place to be distracted.

KILOGRAM (kg)

The kg will change, and the Plank constant will be fixed forever at h = 6.62607015 x 10-34 Js exactly.

METER (m)

No change to the SI unit of length (m)

When this was previously changed, the speed of light was fixed forever at 299,792,458 m/s exactly.

SECOND (s)

No Change to the SI unit of time (s).

MOLE (mol)

The amount of substance (mol) will change by making Avogadro constant NA is 6.02214076 x 1023 mol-1 exactly.  The previously defined mass of carbon 12 will no longer be exactly 0.012 kg/mol.

AMPERE (A)

The SI definition of current (A) will change, the elementary charge of an electron will be fixed forever at e = 1.602176634 x 10-19 C exactly.

KELVIN (K)

The SI unit of temperature (K) will change, the Boltzmann constant will be fixed forever at k = 1.380649 x 10-23 J/K exactly.

CANDELA (cd)

No Change to the SI unit of light (cd).

Of course, many derived units will be affected by the redefinition’s, but great care has been taken so that all changes will lie within the area of uncertainty of the previously defined units, so it shouldn’t make any difference to any previous work.  All these changes will be approved during November 2018 and implemented on world metrology day on 20th May 2019.  No further changes are anticipated in the near or medium future after this date as all artifacts will have been removed and all fundamental units will rely on fundamental constants or atomic data.

# Nanotechnology is too big for NPL

They had put on sandwiches, fresh fruit, cakes, coffee etc.  Nice spread while we waited for the Cambridge crew who arrived before the food had all gone.

We were led into a lecture room with beautiful ceiling roses and old paintings around the walls.  This building used to belong to King William IV back in the day but now its part of NPL.  Our first days lectures were held in the Bushy road entrance but all other days at the main entrance where the iconic glass entrance is.  They started off slowly with introductions to what was to follow the rest of the week.

The lecturers for the day were, Robert Gunn, Stephanie Bell and Ian Robertson.  Our host and Course Director was Andrew Lewis.  I must say that I find it hard to get excited

about the history of the SI units, organisations, tractability and uncertainty but they had regular breaks with coffee, tea, juice, biscuits, fruit and other snacks.  They also showed us down in the cellar where plenty of artifacts were held.  This was very interesting, I wish I had more space for pictures as I took so many every day.  I seem to have become the official photographer of our group as everyone wants me to put the pictures on our shared Google Drive.  They had lots of old light bulbs from the 1900’s which were huge and over complicated.  They had the standard yard that we had in England and preceded the Meter which was held in France.  But one of my highlights was the lecture notes that were sectioned in a folder.  The best that I have ever seen and so accurate; we only had one page change the entire week.

Tuesdays lectures focused on mass.  Its a huge time for metrology at the moment, next year, three of the seven SI units will be redefined in a modern way using fundamental constants rather than artifacts.  The kilogram (kg) is one of them.  At the moment, we have a physical weight made from platinum-iridium alloy since 1883 that everything else has to be ultimately measured against.  This is a single artifact that is very vulnerable.  It also looses weight constantly when it is cleaned.  It gains weight from buildup of hydrocarbons in the air and water vapor in the air until it is cleaned.  Prior to this the kg was defined as the mass of one cubic decimeter of pure water held at 4 degrees Celsius.  But water evaporates, it has surface tension so its difficult to measure, how pure, 4 degrees Celsius exactly etc.  So this definition was immediately replaced by a pure platinum weight in 1799 held inside 3 glass jars.  In 1889 another 40 copies were produced and spread around the world to be used as a local standard.  Every now and then (four times), they all have to be compared with the ultimate standard held in Paris.  The picture above is kg number 18 held at NPL.  At the moment, the kg is only 3 micro-grams accurate because of its variations.  Clearly, we desperately need a better way to define it.  Of course, its not just mass that is affected, the derived units, including pressure and density rely on the kg and if we measure something really light or really heavy like a plane, it has to be in multiples of this standard kg where more accuracy is lost.  It was decided that an accuracy of less than 2 parts in 100,000,000 is required.  There have been two contenders, the Avogadro based kg or the electrical Watt based kg that uses the Plank constant.  The first is a perfectly round sphere made of silicon so that the total number of atoms can be measured and calculated.  This would also redefine the mole at the same time, fixing Avogadro’s constant.  The Watt balance, or Kibble balance after Dr Bryan Kibble from NPL who invented it in 1975, uses electrical energy to balance physical objects and rely s on fixing the Plank constant.  The Kibble balance is the one that will be approved next year and the Plank constant will be fixed forever more.  Although the silicon sphere worked we would still end up with an artifact, which by the way, is the most spherical object ever made.  The beauty of the electrical balance is that one could be made anywhere.  This second one, is the new version that will actually be used in the kg’s redefinition.  Planks constant will be fixed at exactly 6.62606 x 10 -34 expressed in Js on 20th May 2019 which is world metrology day.  Our lecturers for the day were, Stuart Davidson, Ian Robinson, Andy Knott and Kevin Douglas.  Ian Robinson was great friends with Bryan Kibble and worked with him for years before he passed.  It is Ian that continues this work today.  We also managed to hold a conference meeting at lunchtime.

Wednesday’s lectures were all about the redefinition of the meter.  Again, the international prototype meter was an artifact that was redefined in 1960 and again in 1983 by fixing the speed of light to the distance traveled by light in 1/299792458 of a second (in a vacuum), thus fixing the speed of light to exactly 299792458 m/s forever.  Of course, light is electromagnetic radiation which has a frequency.  The frequency of light ranges greatly, of which visible light (that we can see) is only 1 ten trillionth of the total spectrum.  Light of all frequencies travel at the same speed in a vacuum, but vary through any other medium according to the wavelength.  Today, the workhorse of distance measurement is the interferometer that creates interference patterns by splitting light and changing the path distances slightly.  Arthur Schawlow at Stanford University said, “Never measure anything but frequency!” in his Nobel prize speech.  When measuring distances, the greatest enemy is temperature because when temperature changes, size changes.  Also different materials vary by differing amounts with temperature which doesn’t help either.  Its expensive to make a room that doesn’t vary with temperature from 20 plus or minus 0.1 degrees Celsius.  That’s why the eccentric Tim Coveney of NPL has designed the NPL ULTIMATUM.  Just pop in what you want measuring and it will measure it to an accuracy of 1µ / m while holing the temperature constant to 1/10 degree Celsius.  Other lecturers included, Geoffrey Barwood, Andrew Lewis, Ben Hughes and Andrew Yacoot.  In the evening we were all treated to a 3 course meal with drinks at the Kings Head.  What a great day, it just keeps on getting better.

On Thursday we switched our attention to temperature.  Currently, temperature rely s on the triple point of pure water which is 273.16 K (0.01 ·C).  There is only one temperature where water can exist as a gas, liquid and solid at the same time.  This can currently be measured to 50 µK.  As I have mentioned in a previous post, temperature will be redefined as well and the instrument that will be used was made by Paul Morantz that finished working for Cranfield today (end November 2017).  He built this for NPL who are again instrumental in the redefinition of the Kelvin.  The spherical resonator is filled with Argon and the frequency of sound is varied until the fundamental frequency is found inside the resonator.  Because the sphere is made so accurately and the frequency is well known, this frequency will vary with temperature according to the Bolzmann constant so its possible to accurately measure the temperature from the frequency.  To make it even more accurate, the sphere has three slightly different diameters which creates three resonant frequencies.  Our lecturers included, Andrew Yacoot, Robin Underwood, Jonathon Pearce, Radka Veltcheva and Gavin Sutton.  In the evening, I took the train to Waterloo to meet Georgina.  Had a bit of a hitch as my phone has almost expired and refused to work at the critical time when we were trying to make contact.  Just managed to get it going in time otherwise we would never have found each other in the busiest station in Europe.  We had a meal at Wagamama’s but it was freezing.  I really must get myself a jumper or warm coat.

Friday, our final morning looked at the units that are derived from temperature.  Our lecturers were Jonathon Pearce, Helen McEvoy and Stephanie Bell.  They fed us lunch and we were on our way.  Mohammad bought chocolates (we all contributed) for Andrew, our host and Emma and Ronnie who had organised our week.  Andrew was very touched and said that we were the first group ever to buy a gift for them.  We were given our assignment and away we went.  What a week, I wont forget that in a hurry.  We asked Andrew Yacoot who took us for lectures on Wednesday and Thursday to speak at our Conference next

year as he has managed to make an Xray interferometer that can measure distances to a resolution of 24 pm.  To do this he has to hold the temperature stable to less than 1 mK.  Considering that the Silicon atom that he measured is only 192 pm it seems impossible to measure smaller but because its possible it will be done.  It will be used initially for the calibration of optical interferometer’s but I am certain that we will find further uses for it in time.  Its rarely that you ever have the opportunity to meet such an intelligent group of people as this bunch of wonderful mad scientists!