Silver halide and programmable logic controller: what they have in common

1. Step 1. Silver halide — is made of — Silver. A silver salt.
2. Step 2. Silver — uses — Electrical conduction. The best conductor.
3. Step 3. Electrical conduction — is used in — Track circuit. The wheelset closes the circuit.
4. Step 4. Track circuit — has part — Relay. Track relays.
5. Step 5. Relay — evolved into — Programmable logic controller. It replaced relay cabinets.

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