Washing machine and lead–acid battery: what they have in common

1. Step 1. Washing machine — uses — Centrifugal effect. The spin cycle.
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.

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