Electrocardiograph and lead–acid battery: what they have in common

1. Step 1. Electrocardiograph — evolved from — Galvanometer. Einthoven’s string galvanometer.
2. Step 2. Galvanometer — evolved into — Ammeter. An ammeter is a galvanometer with a scale in amperes and a shunt that carries most of the current.
3. Step 3. Ammeter — uses — Direct current. In a school circuit the ammeter shows the direct current from the battery.
4. Step 4. Direct current — is used in — Voltaic pile. It produces direct current.
5. Step 5. Voltaic pile — evolved into — Lead–acid battery. Planté’s battery developed the idea of a chemical current source by making it rechargeable.

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