Recognize electromechanical devices by construction Which electromagnetic device uses carbon brushes and a mechanical commutator as part of its normal operation?

Introduction / Context:
Different electromagnetic devices have distinct internal constructions that reveal their function. Identifying which machines employ brushes and a commutator helps differentiate DC machines from AC machines and from passive actuators like relays and solenoids.

Given Data / Assumptions:

• Commutator: a segmented copper cylinder switched by brushes.
• Brushes: typically carbon blocks that make sliding contact.
• We compare common devices: speaker, DC generator, relay, solenoid.

Concept / Approach:
In DC machines (motors and generators), the commutator mechanically rectifies the internal alternating EMF induced in the armature windings, providing unidirectional current at the terminals. Brushes conduct between the rotating commutator and the external circuit. Other devices listed do not require commutation by brushes: relays and solenoids are stationary actuators; loudspeakers use a voice-coil and permanent magnet without sliding electrical contacts.

Step-by-Step Solution:
Recall DC generator (or motor) anatomy: armature, commutator, brushes, field.Function: convert mechanical to electrical (generator) or electrical to mechanical (motor) with DC at terminals.Match feature: only the DC generator in the list uses brushes and a commutator.Select: “A dc generator.”

Verification / Alternative check:
AC machines use slip rings (not commutators) for rotor currents in wound-rotor designs; speakers, relays, and solenoids have no sliding DC commutation system.

Why Other Options Are Wrong:
Speaker: uses a flexible cone and a voice coil driven by AC.Relay: uses stationary contacts actuated by a magnetic field; no commutator.Solenoid: a coil with a moving plunger; no brushes.

Common Pitfalls:
Confusing slip rings with commutators; slip rings provide continuous AC connection, while commutators segment and rectify.

Final Answer:
A dc generator