Fractional distillation and optical telegraph: what they have in common

1. Step 1. Fractional distillation — is used in — Chemical plant. Separation.
2. Step 2. Chemical plant — has part — Programmable logic controller. Process control automation.
3. Step 3. Programmable logic controller — evolved from — Relay. It replaced relay cabinets.
4. Step 4. Relay — is part of — Electrical telegraph. Relays boosted the signal on long lines.
5. Step 5. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.

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