Dear fellow-nuts,

please don't struggle about the absolute value of volt standards:

there's always an abstract

**definition** of direct and derived SI units, and in most cases a totally different way to practically

*realize* the unit. (called 'mise en pratique')

For the volt, the official SI-definition from 1948 is:

"The (SI) volt is the potential difference between two points on a conductor which carries a constant current of 1 Ampère, if the dissipated power between those two points equals 1 Watt"

You see, that this academical definition really is "1 Volt" (based on kg, m,s).

(The kilo-gram is the only SI unit which is based on 1000 units of a gram, for practical reasons of its realization also, i.e. the kilogram artifact in Sèvres, France)

The realization of the SI Volt is done either by an Hg electrometer, by Clothier at al (1989) basically a balance, measuring the electrical force, uncertainty 0.27ppm, or a design by Funck et. al. (1991), measuring the force on a capacitor plate (0.31ppm uncertainty).

Both experiments are very complicated and clumsy, and might have been performed only once.

The output voltages of those primary standards (SI) can be different from 1V, given by the practical setup.

Then, there is a different

**representation** of the Volt, that is a more easy-to-manage-way to deliver the volt.

Currently this is the Josephson Volt, a few mV in 1972 were amplified by a cryogenic divider to ~ 1V, 1V in 1985 by a Josephson Junction array, and 10V in 1987 by a longer array. (based instead on 2e/h)

Remark: The definition of the

*Representation* of the Volt has been redefined in 1990, but not the

**definition** of the Volt in the SI. The definition from 1948 is still valid!

The uncertainty between two different Josephson experiments can be as low as dV/V = 3e-19 for single JJs and 1.2e-17 for 0.6V arrays if you compare directly on the cryogenic (i.e. quantum) level.

Remember: The uncertainty between the Josephson Volt and the SI is still 0.4ppm!

10V for secondary reference standards has been chosen only for practical reasons in the analogous world (in contrast to the cryogenic world), reasons are: the typical offset of several µV, the limitation to measure volt differences to a few nV only, and because the typical cardinal points 1kV, 100V, 1V and 100mV are symmetrically situated around 10V, so that a 100:1 and a 10:1 divider is sufficient to transfer 10V to all of them.

Anyhow, if one selects a different standard value, e.g. 5V or 7,147V, it's also ok, as there will be always the necessity to make a transfer to other needed calibration points (by KV divider, or by 3458A).

Therefore, the foregoing discussion, which calibration point is the 'correct' value is simply a lack of knowledge of the concept of SI - 'Definition', - ' Realization' and - 'Representation'.

I recommend the lectures 'école de physique, Les Houches, “Quantum Metrology and Fundamental Constants” ', Blaise Jeanneret, "Volt metrology: The Josephson effect and SIS junction arrays":

http://www.metas.ch/LesHouches/downloads/talks/15_Jeanneret.pdf.

Frank