Electronics > Metrology

#1000: My (hi)story of the Weston cell, of the Volt, and of being a volt-nuts

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Dr. Frank:
My collection of Weston cells, all salvaged from the dumpster, started in 1982.
That's marking my personal emergence as a volt-nuts, as for the first time, I collected Volt artefact fix-points, down to the ppm range uncertainty.

These were first measured in 1982, both to 5 1/2 and 6 1/2 digits, about 1.01882(4)V.
This was done by a Fluke 8502A, which was calibrated daily or weekly by means of 335D, 720A and an ovenized Standard Cell, which were frequently calibrated against a 2mV Josephson Junction, from a lab being located just across the street.
So these values were uncertain to about 10ppm, or better.

These unsaturated types, wrapped in tin foil, are from scrapped FLUKE 803B differential VTVMs.
They were used to calibrate the internal 500V reference.

The first one is from Eppley, now reading 1.01795V, that's a rate of about -25ppm/year.

The one on the left side in their box, is a Muirhead 845-C from 1961. Hello - that's my own vintage!
It still reads 1.01798V, about -1295ppm from nominal 1.0193V over 55 years.

A historical look at the Volt (*):
Weston cells defined the Volt until 1948, when the Absolute Volt was defined, and from then on served as mise-en-pratique (representation) for the Volt only.
The SI-Volt was defined in 1960, just before all these cells were made.
This SI-1960-Volt definition is still valid, up to date, and therefore still uncertain in the SI to about 0.3ppm, currently one of the worst defined units.

As there was no precise way at that time to do it better, all main National Standards Institutes had their own Volt, still based on the Weston cells, and the very difficult comparisons between the countries showed differences up to 20ppm.
In fact, there was the American Volt, the German Volt, the British Volt, and so on.

Between 1968 and 1972, the mise-en-pratique changed to using the single Josephson Junction, and Weston cells were used only as working standards.
The Volt at this time was defined by a Josephson constant of KJ(72) = 483594 GHz, so the absolute level of the worldwide Volt was reduced by about -10ppm.
As this JJ Volt was still difficult to transfer to 1V or 10V level, obviously there were still differences between the national labs, still on the order of a few ppm.
The German PTB claimed to be uncertain to 4*10-8, though.

The next time the cells were measured, was on 14th November 1989, another remarkable date in several aspects.

At first, the values were now determined to 8 1/2 digits, so that was obviously the first day, when I got a brand new 3458A at university.

Secondly, on 1st of January 1990, the SI Volt definition was changed again, exactly the day, when I started my PhD physics thesis.
The mise-en-pratique now was defined by the 10V Josephson Junction array, giving about 1000 times better uncertainty worldwide, from 10-6 to 10-9 level.
Coincidentially, the Josephson constant, was increased by about 8ppm, KJ(90) = 483597.9 GHz, bringing the Volt back to nearly the former level.

For my 2 month old instrument this meant, that it lost its calibration to SI Volt over night; the American Volt had to be increased by exactly 9.264ppm, so that my instrument was now reading about 9ppm high. https://www.nist.gov/sites/default/files/documents/calibrations/tn1263.pdf

This didn't matter for my work, as I made relative volt measurements, at much lower uncertainty requirements, only.

Then I salvaged 3 Muirhead 845-D Reference Cells from about the 1970ties, out of some dead gear.
These are also still fine, the one in the middle only decreased about -360ppm over maybe 45 years.

The saturated Standard Cell from 1965 also was measured first in November 1989, 1.01857420V @ 22.5°C, which gives U20°C = 1.018672V (SI-90), and 26 years later about U20°C=1.018639V, so that would be a drift of about -1.24ppm/ year.


I'm awaiting impatiently July of 2017. This will be the Due Date for the final measurements of the Watt-balances in U.S., Canada and UK, and the German Avogadro experiment.
Succesfully determining Plancks constant h within 2*10-8 uncertainty for one, and 5*10-8 between different experiments, will be the pre-condition for the next new definition of the SI-2018.
The definition of the kilogram, the mol and the Kelvin will be changed, and also will finally re-define the SI-Volt.
It will then have zero uncertainty by definition of the quantum Josephson Effect, with h and e having zero uncertainty. This will also be identical to its mise-en-pratique, at about 10-10 uncertainty level to real world, and within 10-19 @ cryogenic world.

I think, at that moment, we'll have to re-write Volt-Nuts, in capital letters.


(*) taken from:
"Volt Metrology: the Josephson Effect and SIS Junctions Arrays", B. Jeanneret

Great 1000th post!  :-+

Good read, even for us non Volt Nuts. 

I found it interesting that your three salvaged Muirhead cells have such a small serial number range.  Obviously the instruments they were salvaged from were purchased in a single lot.

Your deep understanding of this subject is impressive.
What a great collection of Weston cells.

That is probably the reason, the Fluke 732 standards have still a 1.018 V output today?

Nice 100th post. It has been a while since I have looked into the historical aspects of the volt.


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