Railway semaphore e telephone: o que têm em comum

1. Passo 1. Railway semaphore — evoluiu de — Optical telegraph. The semaphore idea.
2. Passo 2. Optical telegraph — evoluiu para — Electrical telegraph. The electrical telegraph replaced semaphore lines.
3. Passo 3. Electrical telegraph — evoluiu para — Telephone. The telephone grew out of attempts to send several signals over telegraph lines (the “harmonic telegraph”).

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