Ans. They are usually covered at their upper part with a coating of electro-deposited copper to insure good contact with the holder.
Comparison of Copper and Carbon Brushes.—Copper brushes tend to tear and roughen the surface of the commutator, while carbon brushes tend to keep the surface smooth. Copper causes more wear of the commutator than carbon. With carbon brushes, the armature may be run in either direction. The resistance of carbon being greater than copper, there is less short circuiting caused by carbon particles than by those of copper.
Fig. 337.—Westinghouse brush holder. It is made of brass, cast in one piece, and of standard sliding type with a shunt of braided copper wire directly connected to a clamp on each brush and to the solid portion of the holder, where it is held by a screw. This shunt relieves the spring of heavy currents. The holder is so arranged as to be easily accessible for adjustment, cleaning and renewal of carbons. Proper tension is provided by spiral strap springs so mounted as to eliminate friction and give uniform pressure over a wide working range. The spring tension is readily adjusted by a simple ratchet arrangement.
Ques. What is the chief merit of carbon brushes?
Ans. They give less sparking than other types.
Ques. How has the construction of carbon brushes been varied?
Ans. Since, for minimum sparking, it is only necessary that the brush have high resistance in the region near its edge, attempts have been made to increase the conductivity of the other portions by combining with the carbon, copper sheets or wires.
Ques. What are the objections to carbon brushes?
Ans. They are easily broken and not being flexible, vibration, or any roughness of the commutator will cause bad contact.