Which of the following devices does not makes use of eddy current for its action? A/An

Analyze the question: We need the device that does NOT rely on eddy currents to work.

Concept: Eddy currents are circulating currents induced in a piece of bulk metal when the magnetic flux through it changes (or when it moves in a magnetic field). Lenz's law says they oppose the change that produced them.

Going through each option:

• A. Galvanometer: In a moving-coil galvanometer, the coil is wound on a metal (aluminium) frame. Eddy currents in this frame give electromagnetic damping, so the pointer settles quickly without oscillating. Eddy currents are used.
• B. Speedometer: In a magnetic speedometer, a rotating magnet induces eddy currents in a metal drum or disc. The drum experiences a drag torque proportional to speed, which moves the pointer. Eddy currents are used.
• C. Induction furnace: A rapidly alternating current in a coil induces large eddy currents in the metal inside. These heat the metal until it melts. Eddy currents are used.
• D. Induction coil: It consists of a primary coil and a secondary coil wound on a soft iron core. It produces a high voltage from a low DC supply by making and breaking the primary current. It works by mutual induction between the coils. Eddy currents here are an unwanted loss, so the core is made of a bundle of laminated iron wires to reduce them. The device's action does not depend on eddy currents.

Answer: D. The induction coil works by mutual induction. Eddy currents are something its design tries to suppress.

Tip: Remember the uses of eddy currents with "GSIF": Galvanometer damping, Speedometer, Induction furnace, and Full braking systems (electromagnetic brakes). Transformers and induction coils use laminated cores to reduce eddy currents.