Question 536Electricity & Electronics - QMED

The correct answer is D. The brush holder labeled 4 in the illustration is of the reaction type. The reaction type brush holder is used in DC generators and motors, where the brushes are held in place by a spring that applies pressure against the commutator. This allows the brushes to follow the commutator as it wears down, maintaining constant contact. The other brush holders labeled 1, 2, and 3 are not of the reaction type, as they do not use a spring-loaded mechanism to maintain brush contact with the commutator.