Coal mine and electrical telegraph: what they have in common

1. Step 1. Coal mine — is used in — Coal. Extraction.
2. Step 2. Coal — is material for — Coal gas. The gas was made by the dry distillation of coal.
3. Step 3. Coal gas — is used in — Traffic light. A gas lamp lit the signal.
4. Step 4. Traffic light — has part — Relay. Relay controllers.
5. Step 5. Relay — is part of — Electrical telegraph. Relays boosted the signal on long lines.

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