The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2·10 –7 newton per meter of length.
As is already illustrated by the formulations "of infinite length" and "...would produce", the text of the definition of the ampere cannot directly serve as an "instruction" for the realization of the unit of current. It defines – in a material-independent way and in accordance with purely theoretical considerations – only the quantity of the electrical units, but not the practical way of their realization. Similar to the definition of the meter, the definition of the ampere only serves to determine a fundamental constant, namely the magnetic field constant µ0. In addition to the light velocity c see meter definition, the electrical field constant ∈ 0 has been determined. When these values and the laws of physics are known, there are many possibilities for realizing absolute values of electrical quantities for calibration purposes. In many national metrology laboratories, this is today done with the aid of quantum effects.
At present, the most exact methods for reproducing the electrical units make use of two quantum effects: the volt is reproduced with the Josephson effect, the ohm with the quantum Hall effect. Both units are linked up with fundamental constants via these quantum effects.
Perhaps in future it will be possible to trace also the ampere itself back to a fundamental constant, namely to that of the elementary charge. Under technical aspects, this is performed in such a way that – with the aid of thin-film technology – structures will be manufactured which are so small that only single electrons can tunnel between them (Single Electron Tunneling, SET). Up to now, the occurring currents are, however, still extremely low. This is why some more scientific efforts will be required until the ampere can finally be reproduced directly with the aid of SET.