Kettle and optical telegraph: what they have in common

1. Step 1. Kettle — evolved into — Electric kettle. A kettle with a heater.
2. Step 2. Electric kettle — has part — Bimetallic strip. Automatic switch-off when the water boils.
3. Step 3. Bimetallic strip — is used in — Circuit breaker. The thermal trip is a bimetal strip.
4. Step 4. Circuit breaker — uses — Electromagnetism. An electromagnetic trip.
5. Step 5. Electromagnetism — is used in — Electrical telegraph. A telegraph receiver is an electromagnet that attracts an armature.
6. Step 6. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.

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