The SI ampere is still defined via the force between two conductors. But that is impractical to realize, so standard labs derive it via Ohm's law. There is some work going on to redefine the SI ampere based on the charge of a proton. But the BIPM members are discussing that for years now, and they don't seem to be in a hurry to do the change.

Naa, that's not correct. The Proton experiment was too unprecise so it's not followed up.

The proposed and really planned change of the SI system is described here:

http://www.bipm.org/en/si/new_si/why.html (Go through the details on the left side)

It begins with the redefinition of the kg by a (1) Si sphere and (2) the Watt balance, then defining fixed (exact) values of e, h and NA.

As soon as the new kg definition can be realized to a better level than the kg prototype in Sèvres (currently looses weight compared to copies), i.e. as soon as those two experiments agree better than 2x 10^-8, this new SI definition will be set active by BIPM. The status of the experiments is checked every two years.

The Ampere would be then automatically defined by counting electrons (e.g. charge pump), and at a first step realized =

*mise en pratique* by the - only then - exact definitions of Volt and Ohm by Josephson and von-Klitzing quantum standards.

Currently, Volt and Ohm are precise within SI to a level of about 3x10^-7 only (!), but can be practically realized to levels of about 10^-19 to 10^-9, depending on whether you compare e.g. different Volt standards on cryogenic level (SQUID) or at room temperature (problem of thermo voltages).

In a second, later step, if the charge pump experiments (counting electrons, SET) some day delivers currents big enough (nA) and without any missing electron counts, it would be possible to close the Metrological Triangle, i.e the values for e and h would be over determined by the measurements of U ~ f x h/e, R ~ h/e^2, I ~ e x f, and their numerical values could be

**measured** exactly depending on the definition of the Second, instead of initially fixing their values arbitrarily.

As the current SI defintion of the Ohm is precise to 10^-7 Until only, such Ohm standards as the venerable Fluke 5450A and others, are fully sufficient.

Frank