The redefinition of the ampere

The ampere is one of the seven base units of the SI, the international system of units. Its definition is linked to mechanical units, especially the unit of mass, the kilogram. In a future system of units, which will be based on the values of fundamental constants, the ampere will be based on the value of the elementary charge e. This paper describes the technical background of the redefinition. Index Terms – SI units, Josephson effect, quantum Hall effect, single electron effect 1. DEFINITION OF THE AMPERE IN THE SI The ampere, the unit of electric current, is the SI base unit for electricity and magnetism. Until the middle of the 20th century, the ampere was defined on the basis of electrolytic processes. The currently valid definition is based on electrodynamics, and reads: 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 newton per metre of length. Figure 1 illustrates the definition of the base unit "ampere". This definition exploits the fact that the electrodynamic force F per conductor length l between two straight parallel conductors which are placed at the distance r and through which a current I flows, is F/l = μ0·(I2/2πr) in vacuum. The numerical value of the magnetic constant μ0 is fixed through the definition of the ampere to μ0 = 4π·10 N/A2 Since the numerical value of the speed of light c has been fixed through the definition of the meter, also the numerical value of the electric constant ε0 is fixed according to the Maxwell relation μ0·ε0·c = l. The practical implementation of this definition – which, in metrology, is called realisation – must be performed with the aid of measuring arrangements that can be realised experimentally. One distinguishes between direct realisation – in which the current is related to a mechanical force – and indirect realisation. The latter is based on Ohm's law I = U/R and the ampere is realised through the realisation of the electric voltage U and the electric resistance R. The fixing of the numerical value of the electric constant ε0, which follows from the definition of the ampere, ensures the link between the indirect realisation of the ampere and its definition. This – still valid – definition of the ampere is based on mechanical quantities. A completely different approach to define the ampere and the electric units is the use of quantum effects in solids. With the aid the Josephson effect [1], the voltage U, and with the aid of the quantum Hall effect [2] the resistance R can be traced back to the elementary charge e and Planck's constant h. Thanks to these effects, the volt, the ohm and, using Ohm's law, also the ampere can be reproduced with highest accuracy. The new definition of the ampere – which has been suggested by the bodies of the Metre Convention – is based on the elementary charge e. The new definition and the state of the art of research on its practical implementation are described in Section 4. Section 5 deals with consistency tests aiming to ensure the implementation of the new ampere definition. The article closes with an outlook. 2. REALISATION OF THE AMPERE IN THE SI 2.1. Direct realisation The direct realisation of the ampere, based on the definition, is only of historical interest today. The Figure 1 Schematical representation of the ampere definition URN (Paper): urn:nbn:de:gbv:ilm1-2011iwk-151:3