Which of the following devices is associated with alternating currents only?

Analyze the question: We need the device that works with alternating current (AC) only, meaning it cannot work with steady direct current (DC).

Concept:

• A transformer works by mutual induction. The changing current in the primary coil produces a changing magnetic flux, which induces an e.m.f. in the secondary coil. Steady DC gives a constant flux, so no e.m.f. is induced. A transformer therefore works only with AC.
• An induction coil changes low DC voltage into a very high voltage. It uses a make-and-break interrupter to keep interrupting the DC supply, so it is powered by DC, not AC only.
• A moving coil galvanometer detects small DC currents. The deflection depends on the direction of the current, so it is for DC.
• A lead-acid accumulator is a secondary cell. It supplies and is charged with DC.

Step by step:

1. Lead-acid accumulator: DC only, so eliminate D.
2. Moving coil galvanometer: DC, so eliminate C.
3. Induction coil: runs on a DC battery with an interrupter, so eliminate B.
4. Step-up transformer: needs a changing current and so works only on AC. This matches the question.

Answer: A. Step-up transformer

Tip: Remember "transformer = AC only." If a device needs a changing flux without any mechanical interrupter, it needs AC. The induction coil is a common trap, because it gives high voltage like a transformer but is fed by DC through its interrupter.