Photodiode and submarine: what they have in common

1. Step 1. Photodiode — is used in — Fax. Scanning with a photocell.
2. Step 2. Fax — uses — Electrochemical reaction. The paper darkened under current.
3. Step 3. Electrochemical reaction — is used in — Lead–acid battery. A reversible reaction of lead and lead dioxide in sulfuric acid.
4. Step 4. Lead–acid battery — is part of — Submarine. Underwater, electric motors run on batteries.

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