Railway signal lamp and telephone: what they have in common

1. Step 1. Railway signal lamp — is part of — Railway semaphore. A night signal.
2. Step 2. Railway semaphore — evolved from — Optical telegraph. The semaphore idea.
3. Step 3. Optical telegraph — evolved into — Electrical telegraph. The electrical telegraph replaced semaphore lines.
4. Step 4. Electrical telegraph — evolved into — Telephone. The telephone grew out of attempts to send several signals over telegraph lines (the “harmonic telegraph”).

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