Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.
The ampere (
SI defines the ampere in terms of other base units by measuring the electromagnetic force between electrical conductors carrying electric current. The earlier
In the future, the SI definition may shift back to charge as the base unit, with the coulomb defined in terms of the
SI defines ampere as follows:
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one
metreapart in vacuum, would produce between these conductors a force equal to ×10−7 2 newtonsper metre of length.   
The SI unit of charge, the
In general, charge Q is determined by steady current I flowing for a time t as Q = It.
Constant, instantaneous and average current are expressed in amperes (as in "the charging current is 1.2 A") and the charge accumulated, or passed through a circuit over a period of time is expressed in coulombs (as in "the