Galvanometer and tank: what they have in common

1. Step 1. Galvanometer — has part — Permanent magnet. A magnet.
2. Step 2. Permanent magnet — is part of — Compass. The needle is a magnet.
3. Step 3. Compass — evolved into — Gyrocompass. A compass without a magnet.
4. Step 4. Gyrocompass — is part of — Ironclad. Steel ships distort a magnetic compass.
5. Step 5. Ironclad — has part — Armour. An armour belt.
6. Step 6. Armour — is part of — Tank. Armour protects the crew.

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