The base SI unit of current; symbol A, often called amp, named after André Ampère; defined as the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible cross-section, and placed 1 metre apart in vacuum, would produce a force equal to 2 × 10?7 N/m.
The ampere (symbol: A) is the SI base unit of electric current.
Definition
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 metre of length.
The ampere is defined first (it is a base unit, along with the metre, the second, and the kilogram), without reference to the quantity of charge. The unit of charge, the coulomb, is defined to be the amount of charge displaced by a one ampere current in the time of one second.
Explanation
Because it is a base unit, the definition of the ampere is not tied to any other electrical unit.
The ampere is most accurately realized using an ampere balance, but is in practice maintained via Ohm's Law from the units of voltage and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson junction and the quantum Hall effect, respectively.
The unit of electric charge, the coulomb, is defined in terms of the ampere: one coulomb is the amount of electric charge (formerly quantity of electricity) carried in a current of one ampere flowing for one second. One ampere of current (I) is equal to a flow of one coulomb of charge (Q) per second of time (t):
Proposed future definition
Since a coulomb is approximately equal to 6.24150948×10 elementary charges, such as electrons, moving past a boundary in one second.
As with other SI base units, there have been proposals to redefine the kilogram in such a way as to define some presently measured physical constants to fixed values.
CIPM recommendation
International Committee for Weights and Measures (CIPM) Recommendation 1 (CI-2005): Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants
The International Committee for Weights and Measures (CIPM),
approve in principle the preparation of new definitions and mises en pratique of the kilogram, the ampere and the kelvin so that if the results of experimental measurements over the next few years are indeed acceptable, all having been agreed with the various Consultative Committees and other relevant bodies, the CIPM can prepare proposals to be put to Member States of the Metre Convention in time for possible adoption by the 24th CGPM in 2011;
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