more pairs of brushes that touch the commutator; the brushes connect an external source of electric power to the rotating armature. The rotating armature consists of one or more wire coils wound around a laminated, magnetically "soft" ferromagnetic core. Current from the brushes flows through the commutator and one winding of the armature, making it a temporary magnet (an electromagnet). The magnetic field produced interacts with a stationary magnetic field produced by either PMs or another winding (a field coil), as part of the motor frame. The force between the two magnetic fields rotates the shaft. The commutator switches power to the coils as the rotor turns, keeping the poles from ever fully aligning with the magnetic poles of the stator field, so that the rotor keeps turning as long as power is applied. Many of the limitations of the classic commutator DC motor are due to the need for brushes to maintain contact with the commutator, creating friction. The brushes create sparks while crossing the insulating gaps between commutator sections. Depending on the commutator design, the brushes may create short circuits between adjacent sections—and hence coil ends. Furthermore, the rotor coils' inductance causes the voltage across each to rise when its circuit opens, increasing the sparking. This sparking limits the maximum speed of the machine, as too-rapid sparking will overheat, erode, or even melt the commutator. The current density per unit area of the brushes, in combination with their resistivity, limits the motor's output. Crossing the gaps also generates electrical noise; sparking generates RFI. Brushes eventually wear out and require replacement, and the commutator itself is subject to wear and maintenance or replacement. The commutator assembly on a large motor is a costly element, requiring precision assembly of many parts. On small motors, the commutator is usually permanently integrated into the rotor, so replacing it usually requires replacing the rotor. While most commutators are cylindrical, some are flat, segmented discs mounted on an insulator. Large brushes create a large contact area, which maximizes motor output, while small brushes have low mass to maximize the speed at which the motor can run without excessive sparking. (Small brushes are desirable for their lower cost.) Stiffer brush springs can be used to make brushes of a given mass work at a higher speed, despite greater friction losses (lower efficiency) and accelerated brush and commutator wear. Therefore, DC motor brush design entails a