The armature is connected to a commutator and brushes. Metal brushes and a flat commutator switch power to the rotor coils. The set of traceless Hermitian complex matrices with Lie bracket given by times the commutator. The commutator is essentially a rotary switch. A dynamo is an electrical generator that creates direct current using a commutator. The ends of the wire winding are connected to a commutator. A DC motor is usually supplied through a slip ring commutator as described above. A dynamo has the disadvantages of a mechanical commutator. When considering pairs of observables, an important quantity is the "commutator". Each conducting segment of the commutator is insulated from adjacent segments. Brushes must not be confused with a commutator. They use slip rings having greatly extended brush life over a commutator. Typically the brush is wide enough to span 2.5 commutator segments. A spring is typically used with the brush, to maintain constant contact with the commutator. The different brush types make contact with the commutator in different ways.