In storing magnets, the keeper is used to

Step 1: Analyze the question The question asks why a keeper (a soft iron bar) is placed across the ends of magnets when they are stored.

Step 2: The concept When a bar magnet is stored alone, its free poles create a magnetic field that acts opposite to the magnet's own magnetization inside it. Over time, this makes the magnet gradually lose its strength. This is called self-demagnetization.

To prevent this:

• Bar magnets are stored in pairs, with unlike poles side by side (N next to S), and soft iron keepers are placed across the ends.
• For a horseshoe magnet, a single soft iron keeper is placed across the two poles.

The keeper becomes magnetized by induction, which forms a closed loop of magnetic flux through the magnet and keeper. With no free poles, the demagnetizing field is greatly reduced, so the magnet keeps its strength.

Step 3: Evaluate the options

• A. Cancel the effect of the earth's magnetism: Incorrect. The keeper doesn't cancel the earth's field. Storing magnets in a north-south direction is a separate precaution.
• B. Reduce self demagnetization: Correct. The closed flux loop removes free poles and preserves the magnetism.
• C. Protect from stray electric field: Incorrect. Electric fields don't demagnetize magnets.
• D. Increase the strength of the magnet: Incorrect. The keeper only preserves strength. It does not make the magnet stronger.

Tip: Remember "Keeper = Keeps the magnetism". A keeper keeps the strength by closing the magnetic circuit, so it prevents loss and never adds strength. Other things that demagnetize a magnet are heating, hammering, and an alternating current, so avoid these as well.