The measurement of electric current is chiefly based on the-19th century by Hans Christian Oersted and Michael Faraday of the relationship between electricity and magnetism. The fundamental association is the phenomenon, first reported by Oersted in 1820, that an electric current passing through a conductor produces a magnetic field, which in turn exerts a force on other gives a measure of the current. This fact is also used in the current that gives rise to a force exerted by one on the other gives a measure of the current. This fact is also used in the definition of the unit of current. This fact is also used in the definition of the unit of current; one ampere is the current that gives rise to a force of 2 x 10-7
In practical measuring instruments that make use of this phenomenon, the conductors consist of two coils in different ways, such electrodynamometer as Ammeters, voltmeters, and wattmeters may be constructed, as well as a class of instrument known as Ferraris meters. Measurement with an electrodynamometer is based on determining the force acting between the two coils through which current flows. This is effected by measuring the deviation from equilibrium of a free turning coil opposed by a small spring. In Ferraris meter the two coils are fixed with respect to a piece of metal that is free to turn, and the measurement is based on the eddy currents induced in the metal. In all of these two coil instruments, the magnetic field is weak when the current intensity is low, so that these meters are not very sensitive. However they can measure both direct and alternating currents.
Another widely used electromagnetic current measurement instrument is the moving coil meter, or Galvanometer, in which the current to be measured flows through a coil suspended in a strong magnetic field induced by a permanent magnet rather than by the current itself. Such instruments measure only direct current.
