Barometer and voltaic pile: what they have in common

1. Step 1. Barometer — is made of — Mercury. A column of mercury.
2. Step 2. Mercury — is part of — Hydrometer. Old hydrometers used mercury or lead shot as the weight to keep them upright.
3. Step 3. Hydrometer — is used in — Lead–acid battery. A car battery's charge is checked with a hydrometer by the density of its acid.
4. Step 4. Lead–acid battery — evolved from — Voltaic pile. Planté’s battery developed the idea of a chemical current source by making it rechargeable.

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