Exposure meter and steam locomotive: what they have in common

1. Step 1. Exposure meter — has part — Photodiode. A photocell.
2. Step 2. Photodiode — uses — P–n junction. Light generates a current in the p–n junction.
3. Step 3. P–n junction — is used in — IGBT. A transistor structure with p–n junctions.
4. Step 4. IGBT — is part of — Electric locomotive. Traction converters.
5. Step 5. Electric locomotive — has part — Railway coupling. An electric locomotive pulls its wagons through a coupling.
6. Step 6. Railway coupling — is part of — Steam locomotive. A locomotive hooks up its wagons with a coupling.

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