Which material are use to make an Electrical Brushes? Why? In Hindi Interview Question

Which material are use to make an Electrical Brushes? Why? In Hindi Interview Question.
Some student know about Electrical brushes and make but doesn't know about reason behind that.
These type of question ask in interview and check the deepest knowledge about Electrical machine and material knowledge.
In that Electrical carbon are use to brushes in Electrical machine.
Behind that
Electrical carbon are smoother than other conductor and reduce sparking in friction of rotor and Brushes
And other is negative temperature coefficient
And that's why brush loss is constant.

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