Radio navigation e railway semaphore: o que têm em comum

1. Passo 1. Radio navigation — utiliza — Radio. Radio beacons.
2. Passo 2. Radio — utiliza — Morse code. The first radio transmissions were in Morse code.
3. Passo 3. Morse code — é utilizado em — Electrical telegraph. Morse encoding.
4. Passo 4. Electrical telegraph — evoluiu de — Optical telegraph. The electrical telegraph replaced semaphore lines.
5. Passo 5. Optical telegraph — evoluiu para — Railway semaphore. The semaphore idea.

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