E-reader and railway semaphore: what they have in common

1. Step 1. E-reader — has part — Electronic ink. An electronic ink screen.
2. Step 2. Electronic ink — uses — Electromagnetism. The pigment moves in an electric field.
3. Step 3. Electromagnetism — is used in — Electrical telegraph. A telegraph receiver is an electromagnet that attracts an armature.
4. Step 4. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.
5. Step 5. Optical telegraph — evolved into — Railway semaphore. The semaphore idea.

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