Mercury and programmable logic controller: what they have in common

1. Step 1. Mercury — is part of — Hydrometer. Old hydrometers used mercury or lead shot as the weight to keep them upright.
2. Step 2. Hydrometer — is used in — Lead–acid battery. A car battery's charge is checked with a hydrometer by the density of its acid.
3. Step 3. Lead–acid battery — uses — Sulfuric acid. The electrolyte.
4. Step 4. Sulfuric acid — is used in — Chemical plant. The first chemical plants produced sulfuric acid and soda.
5. Step 5. Chemical plant — has part — Programmable logic controller. Process control automation.

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