Ammeter and tunnelling shield: what they have in common

1. Step 1. Ammeter — uses — Direct current. In a school circuit the ammeter shows the direct current from the battery.
2. Step 2. Direct current — is used in — Inverter. An inverter turns a battery's direct current into alternating current.
3. Step 3. Inverter — has part — IGBT. The power switches of inverters.
4. Step 4. IGBT — is part of — Electric locomotive. Traction converters.
5. Step 5. Electric locomotive — is used in — Metro. The first lines were steam-operated; electric trains since 1890.
6. Step 6. Metro — uses — Tunnelling shield. Metro tunnels were built with shields.

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