Signal wire and electrical grid: what they have in common

1. Step 1. Signal wire — is part of — Railway semaphore. A cable from the control post.
2. Step 2. Railway semaphore — evolved into — Traffic light. The first traffic light was a railway-style semaphore.
3. Step 3. Traffic light — uses — Coal gas. A gas lamp lit the signal.
4. Step 4. Coal gas — is made of — Coal. The gas was made by the dry distillation of coal.
5. Step 5. Coal — is used in — Power station. The first stations ran on coal.
6. Step 6. Power station — is part of — Electrical grid. Power stations feed the grid.

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