Safety valve and lead–acid battery: what they have in common

1. Step 1. Safety valve — is part of — Autoclave. A safety valve releases pressure.
2. Step 2. Autoclave — has part — Manometer. The gauge shows whether the steam pressure in the chamber is high enough to sterilize.
3. Step 3. Manometer — evolved into — Blood pressure monitor. A blood pressure monitor is a manometer connected to a cuff: it measures the air pressure that squeezes the artery.
4. Step 4. Blood pressure monitor — is made of — Mercury. A column of mercury.
5. Step 5. Mercury — is part of — Hydrometer. Old hydrometers used mercury or lead shot as the weight to keep them upright.
6. Step 6. Hydrometer — is used in — Lead–acid battery. A car battery's charge is checked with a hydrometer by the density of its acid.

Open Evothings