Why Carbon is Selected as the medium?

About Carbon Brushes:
1. Carbon Brush	2. Why Carbon is Selected as the medium?
3. Current Density in Carbon Brush	4. Why Carbon Brushes are split?
5. What is Patina?

In all modern technology for transmitting current from a stationary body to a rotating body carbon brush is used. The main reasons for using carbon are as follow:

a) First of all is its conductivity which is as like as metal.

b) The main disadvantage metal is that in some temperature metal melts. Most often metal welded because of this property. On the other hand carbon does not melt. It does not hold any liquid state. It vaporize directly from solid state. It is another very good advantage of using carbon brush. No probability to be welded with commutator or slipring. Also its evaporation  temperature is also too high. It is almost 3000 to 4000 degree Celsius.

c) Carbon is lighter than many metal which also make it's inertia much low and because of this low inertia carbon brush can move easily and smoothly over little rough surface without causing any trouble. 

d) Another important quality is carbon is self lubricating which means low friction. As temperature rises unlike any other metal carbon friction coefficient decreases making it suitable for using in rotating machinary.

e) Chemically carbon is stable and it's expansion is also negligible.

These are some reasons which make carbon as a best choice.