Commutator Coils Shaft Figure 9-47.  A drum-type armature.  Figure 9-46.  Generator field frame. Figure 9-48.  Commutator with portion removed to show construction. Mica V-ring Front V-ring Commutator bar Mica Iron shell Back V-ring with mica inner and outer rings for insulation Slots Tightening nut Iron ring Commutator bars Mica insulation between bars 9-31 Figure 9-48  shows a cross-sectional view of a typical  commutator. The commutator is located at the end of an  armature and consists of copper segments divided by a thin  insulator. The insulator is often made from the mineral mica.  The brushes ride on the surface of the commutator forming the  electrical contact between the armature coils and the external  circuit. A flexible, braided copper conductor, commonly  called a pigtail, connects each brush to the external circuit. The  brushes are free to slide up and down in their holders in order  to follow any irregularities in the surface of the commutator.  The constant making and breaking of electrical connections  The commutator surface is highly polished to reduce friction  as much as possible. Oil or grease must never be used on a between the brushes and the commutator segments, along with  the friction between the commutator and the brush, causes  brushes to wear out and need regular attention or replacement.  For these reasons, the material commonly used for brushes is  high-grade carbon. The carbon must be soft enough to prevent  undue wear of the commutator and yet hard enough to provide  reasonable brush life. Since the contact resistance of carbon is  fairly high, the brush must be quite large to provide a current  path for the armature windings. Commutators