Why is the Copper Wire in a Carbon Brush Connec....

I suspect that you have either remembered the question incorrectly or that this translation into English is incorrect. What you have written here does not make grammatic sense in English. Possibly the point of this interview question was to see how you would respond to an ambiguous, confusing question instead of testing your knowledge of a carbon brush.

You will find if you crush a carbon brush that the pigtail is embedded in the carbon / graphite compound

Look for Morganite, LeCarbon or The Electric Bush Company on google.

Can we use aluminium or gold or silver instead of carbon brush?

Yes, but not for long. Carbon brushes are designed to wear out before any damage is done to other components. They are relatively easy to replace, for this reason.

there is no special reason for copper wire in such a way. it can can be connected At any place on the carbon brush it is connected according to the mounting and inserting possessions of the brush. for the brushes in low wattage motors copper wire is connected from the centre of the carbon brush to the metal plate with spring tension. for the brushes in high wattage motors copper wire is connected side of the brush

Part 1:

Pigtail connection in a carbon brush is connected from the centre of carbon brush; but not from the centre; as the pigtail is to be inserted deep into the carbon body; so that it carry forward all the current collected by the carbon brush. Also insertion of pigtail at the centre unlike the connection done by attaching at the surface, will be a low resistance joint and also secure one; therefore resulting in lower resisrance power loss at the joint.

Part 2:

Earlier dc machines; had copper breads for collecting current from the commutator; however; they are damaging to the commutator. Also, commutation (reversal of current in the armature conductor, passing under the carbon brush) is assisted by carbon brushes better as compared to copper brushes. This is because; higher resistance of carbon brushes as compared to copper conductors in the motors; helps in transfer of current from one direction to another; when the motor is operating. This in addition to the commutating winding on the stator of the motor; helps to increase the rating of the motor; for a given size/ frame.

Carbon brushes also offers less friction between commutator and carbon; therefore the commutator which is costly item in a dc motor is not subjected to damage; and therefore offers long life to the motor.

Therefore, carbons very much integral part; for the operation in a motor

Thanks Yousef, Lyn and Raghurama for your answers. Let this be the end of this thread.