Signal wire e rede elétrica: o que têm em comum

1. Passo 1. Signal wire — faz parte de — Railway semaphore. A cable from the control post.
2. Passo 2. Railway semaphore — evoluiu para — Traffic light. The first traffic light was a railway-style semaphore.
3. Passo 3. Traffic light — utiliza — Coal gas. A gas lamp lit the signal.
4. Passo 4. Coal gas — é feito de — Carvão mineral. The gas was made by the dry distillation of coal.
5. Passo 5. Carvão mineral — é utilizado em — Power station. The first stations ran on coal.
6. Passo 6. Power station — faz parte de — Rede elétrica. Power stations feed the grid.

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