Carousel and voltaic pile: what they have in common

1. Step 1. Carousel — uses — Centrifugal effect. Rotation.
2. Step 2. Centrifugal effect — is used in — Cream separator. Separation by density.
3. Step 3. Cream separator — works like — Hydrometer. Both rely on density: a separator removes the lighter cream, a hydrometer checks by density whether milk has been watered down.
4. Step 4. Hydrometer — is used in — Lead–acid battery. A car battery's charge is checked with a hydrometer by the density of its acid.
5. Step 5. 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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