In which of the following activities is an induction coil not useful?

Analyzing the question: We need the activity where an induction coil is not useful.

Concept: An induction coil is a device that produces very high voltage pulses from a low-voltage DC source. It works like this:

• A low DC voltage is applied to a primary coil (few turns of thick wire) wound around a soft iron core.
• An interrupter (make-and-break contact) repeatedly cuts the current, so the magnetic flux changes rapidly.
• The secondary coil (many turns of thin wire) then has a very high EMF induced in it, according to Faraday's law.

Because it gives high-voltage sparks and pulses, it is used for:

• Investigating high voltages (A), since it is a source of high voltage.
• Studying electric discharge through gases (B), since the high voltage ionises gas in discharge tubes.
• Operating X-ray tubes (C), since X-ray tubes need tens of thousands of volts (older X-ray machines used induction coils).

Option D: A.C. power transmission. This is done using transformers, not induction coils. Power transmission uses an alternating supply that is stepped up for transmission and stepped down for use. An induction coil needs a DC input with an interrupter, and it produces intermittent pulses. These are unsuitable for efficient, continuous power transmission.

Solution summary:

• A: useful ✔
• B: useful ✔
• C: useful ✔
• D: not useful ✘

Answer: D

Exam tip: Remember "Induction coil = DC in, high-voltage pulses out (with interrupter)", used for sparks, discharge tubes and X-rays. "Transformer = AC in, AC out", used for power transmission. If the question mentions A.C. power transmission, think transformer.