Tunnelling shield and lead–acid battery: what they have in common

1. Step 1. Tunnelling shield — is used in — Metro. Metro tunnels were built with shields.
2. Step 2. Metro — uses — Electric locomotive. The first lines were steam-operated; electric trains since 1890.
3. Step 3. Electric locomotive — has part — IGBT. Traction converters.
4. Step 4. IGBT — is part of — Inverter. The power switches of inverters.
5. Step 5. Inverter — is part of — Uninterruptible power supply. An inverter.
6. Step 6. Uninterruptible power supply — has part — Lead–acid battery. A battery.

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