An a.c. generator can be converted into a d.c. generator by replacing

Analyze the question: It asks what change turns an a.c. generator into a d.c. generator. The key is how the current is taken from the rotating coil to the external circuit.

Concept:

• A coil rotating in a magnetic field produces an alternating emf, because the direction of the induced current reverses every half rotation.
• A.c. generator: the coil ends connect to two slip rings, one ring per end. Each brush always touches the same ring, so the output stays alternating.
• D.c. generator: the coil ends connect to a split-ring commutator (a ring cut into two halves). Every half rotation, each half switches from one brush to the other. This reverses the connection at the same moment the coil's current reverses, so the current in the external circuit always flows in one direction. The output is pulsating but unidirectional d.c.

Checking the options:

• A. Replacing slip rings with split-ring commutators is exactly the change needed. ✔
• B. Armatures are the coils, not a current-collecting device, so this makes no sense.
• C. This does the reverse and would give an a.c. generator.
• D. Brushes are still needed to collect current, so they can't replace the split rings.
• E. The brushes are not replaced. The slip rings are what get replaced by the commutator.

Answer: A, the slip rings are replaced with split-ring commutators.

Exam tip: Remember "Slip rings = Same direction reversed, i.e. a.c." and "Commutator = Corrects direction, i.e. d.c." A commutator commutes (reverses) the connections so the output stays one-way.