[color=blue][b]Two carbon brushes are made to press lightly against the commutators.[/b][/color]
The motor features a external magnet (called the stator because itís fixed in place) and an turning coil of wire called an armature ( rotor or coil, because it rotates). The armature, carrying current provided by the battery, is an electromagnet, because a current-carrying wire generates a magnetic field; invisible magnetic field lines are circulating all around the wire of the armature.
The key to producing motion is positioning the electromagnet within the magnetic field of the permanent magnet (its field runs from its (top) south to (bottom) north poles). The armature experiences a force described by the left hand rule. This interplay of magnetic fields and moving charged particles (the electrons in the current) results in the magnetic force (depicted by the green arrows) that makes the armature spin because of the torque. Use the slider current I to see what happens when the flow of current is reversed. The checkbox current flow & electron flow alows different visualization since I = d(Q)/dt and Q= number of charge*e. The Play & Pause button allows freezing the 3D view for visualizing these forces, for checking for consistency with the left hand rule .
A swing back and fro motion (maybe ? = 0o to 180o and 0o) is all you would get out of this motor if it weren't for the split-ring commutator ó the circular metal device split into parts (shown here in red and blue with a gap of ?1 and ?2) that connects the armature to the circuit. Electricity flows from the positive terminal of the battery through the circuit, passes through a copper brush to the commutator, then to the armature.
When the current is positive, current runs through ABCD as shown in the diagram (select the checkbox labels?), a +y direction force would act on AB. An -y direction force would act on CD. Taking moments about the axle, reveals a resultant torque T = Fmag*AD*cos? acts on the coil loop. The coil loop rotates in an clockwise manner (view from battery side) until it reaches the ? = 80o position (assuming that split ring angle are default at ?1 = -80o and ?2 = 80o). At this ? = 80+o position, the current is cut off. However, the momentum of the loop carries it past the horizontal position until the coil loop reaches ? = 110o position. Contact between loop and split ring commutator is established again and the current in the coil loop is now reversed. A -y direction force now acts on AB while a +y direction force acts on CD. The rotation motion is reinforced clockwise (view from battery side) as ? continues to rotate from 110o to 260o. At this ? = 260+o position, the current is cut off. However, the momentum of the loop carries it past the horizontal position until the coil loop reaches ? = 280o position. Contact between loop and split ring commutator is established again and the current in the coil loop is now reversed back to same as from ? = 280o to 80o. A a +y direction force would act on AB. An -y direction force would act on CD and the loop reaches ? = 0o . The cycle repeats after ? = 0o allowing the armature to experience torque in the reinforced direction at the right time to keep it spinning.
The purpose of the commutator is to reverses the direction of the current in the loop ABCD for every half a cycle.