Strain gauge and lead–acid battery: what they have in common

1. Step 1. Strain gauge — is part of — Digital blood pressure monitor. A pressure sensor.
2. Step 2. Digital blood pressure monitor — evolved from — Blood pressure monitor. An automatic blood pressure monitor.
3. Step 3. Blood pressure monitor — is made of — Mercury. A column of mercury.
4. Step 4. Mercury — is part of — Hydrometer. Old hydrometers used mercury or lead shot as the weight to keep them upright.
5. Step 5. 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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