Why are brushes made from carbon in brush gear for DC machines?

Brushes made from carbon are commonly used in brush gear for DC machines primarily because carbon effectively reduces friction at the commutator. When the brushes make contact with the commutator, these carbon brushes allow for a smooth, sliding action. The physical properties of carbon provide a good combination of electrical conductivity and lower wear at the contact points, which is essential for minimizing heating and providing prolonged life to both the brushes and the commutator.

While the other options may describe some benefits of carbon brushes in general, they do not encapsulate the core reason for their usage in this specific context. The ability of carbon to mitigate friction and wear is critical for the reliable operation of DC machines, ensuring efficient performance and longevity of the components involved in their electrical circuits.