Public address system and voltaic pile: what they have in common

1. Step 1. Public address system — has part — Loudspeaker. Loudspeakers.
2. Step 2. Loudspeaker — is part of — Transistor radio. A loudspeaker.
3. Step 3. Transistor radio — has part — Dry cell. Batteries.
4. Step 4. Dry cell — evolved from — Voltaic pile. A convenient form of the galvanic cell.

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