Railway semaphore und telephone: was sie gemeinsam haben

1. Schritt 1. Railway semaphore — entwickelte sich aus — Optical telegraph. The semaphore idea.
2. Schritt 2. Optical telegraph — entwickelte sich zu — Electrical telegraph. The electrical telegraph replaced semaphore lines.
3. Schritt 3. Electrical telegraph — entwickelte sich zu — Telephone. The telephone grew out of attempts to send several signals over telegraph lines (the “harmonic telegraph”).

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