The Motor Principle: when a current-carrying conductor is located in an external magnetic field perpendicular to the conductor, the conductor experiences a force perpendicular to itself and to the external magnetic field.

The right-hand rule for force on a conductor can be used to determine the direction of the force experienced on the conductor: if the right thumb points in the direction of the current in the conductor and the fingers of the right hand point in the direction of the external magnetic field, then the force on the conductor is directed outward from the palm of the right hand.

The motor principle is used to form a precise definition of ampere. 1 ampere is the amount of current flowing through two straight parallel conductors 1 meter apart in a vacuum which produces a force 2 × 10^-7 newtons per meter of conductor.

Analog electric meters (i.e., galvanometers, ammeters, voltmeters) operate on the motor principle. Electric motors are an important application of the motor principle. An electric motor consists of a permanent external field magnet (stator) and a coiled conducting ammeter (rotor) which is free to rotate within the field magnet. Brushes and a commutator (designed differently if A.C. or D.C. current is supplied to the armature) connect the armature to an external voltage source. The speed of rotation of a motor depends on the amount of current flowing through it, the number of coils on the armature, the strength of the field magnet, the permeability of the armature, and the mechanical load connected to the shaft.