Hydrometer and electrical grid: what they have in common

1. Step 1. Hydrometer — is used in — Lead–acid battery. A car battery's charge is checked with a hydrometer by the density of its acid.
2. Step 2. Lead–acid battery — is part of — Uninterruptible power supply. A battery.
3. Step 3. Uninterruptible power supply — is part of — Data centre. Backup power.
4. Step 4. Data centre — uses — Electrical grid. Power.

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