Hi group,

What happen in 1970 that caused a shift in the Volt?

Regards,

Jay_Diddy_B

That's no volt shift, that's a redefinition of the S.I. volt, and a different

*mise en pratique*.

Until 1972, the national volts were defined and realized by Weston cells, each having a nominal value of about 1.018638 Volt.

Afaik, there also was no precise definition of the Ohm at that time, and the AmpĂ¨re definition also was not so good to be realized, so the S.I. volt was very unprecise in terms of the base units.

As a consequence, each of the different metrology institutes had their own volt, (because the chemistry of every Weston cell always is a bit different, but the assigned value was always assumed to be the nominal one), and all differed and varied , as it is obvious in the curve.

In 1972, the single junction Josephson volt delivered an independent definition and realization, only based on the definition of h, e and the second.

Latter constants of nature were already relatively precisely known, and so was the definition, then. Anyhow, in 1972 the responsible BIPM conference probably had chosen the wrong value for the Josephson constant, off by about -7ppm, or so.

But as a single junction delivered a few mV only, the realization (mise en pratique) was also not very precise, on the order of about 10nV / 2mV ~ 5ppm, I would bet.

You can easily see, that consecutive measurements in one metrology institute already varied by several ppm.

Until 1990, the JJ array (10V) had been developed, which greatly increased the uncertainty of the realization of the volt (factor of about 5000 better), and also e and h meanwhile had been determined more precisely.

So in 1990, the redefinition gave a much more precise value for the Josephson constant, a much more precise mise en pratique, and just recently, then, the von Klitzing Hall effect delivered the quantum Ohm. So the realization of Volt and Ohm was uncertain to better than 1e-9, but within the S.I. both were still unprecise to around 0.2 ..0.3 ppm, and that's still the situation of today.

The next step will probably happen in 2018. Then, Volt and Ohm may get exact values in the S.I., i.e. zero uncertainty for the Josephson and Klitzing constants, as then e and h will be 'exact', as is already the second.

Frank