A continuous current supplied by batteries is very suitable to operate electromagnets with immutable poles, but for the operation of the motor periodic polarity changes are necessary and something has to be done to reverse the direction of current at the appropriate times. In its simplest form, thrust ball bearings a commutator has two curved copper plates secured in isolation on the rotor shaft, the ends of the coil winding are soldered on these plates. The electric current ‘arrives’ through one of the brushes and ‘enters’ the switch plate, passing through the rotor coil, coming out of the other plate of the switch and returning to the source by the other brush.
In most DC electric motors, the rotor is an ‘electromagnet’ that rotates between the poles of stationary permanent magnets. To make this electromagnet more skilful the rotor contains an iron core, which becomes strongly magnetized as the current flows through the coil. The rotor will rotate as long as this current reverses its direction of travel each time its poles reach the opposing poles of the stator. The most common way to give these reversals is to use a switch.