Binocular eyepiece head and lead–acid battery: what they have in common

1. Step 1. Binocular eyepiece head — is part of — Operating microscope. Eyepieces for the surgeon.
2. Step 2. Operating microscope — has part — LED. An illuminator.
3. Step 3. LED — is part of — Torch (flashlight). Modern torches use LEDs.
4. Step 4. Torch (flashlight) — has part — Dry cell. Batteries.
5. Step 5. Dry cell — evolved from — Voltaic pile. A convenient form of the galvanic cell.
6. Step 6. 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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