Railway signal lamp and voltaic pile: what they have in common

1. Step 1. Railway signal lamp — is part of — Railway semaphore. A night signal.
2. Step 2. Railway semaphore — evolved into — Traffic light. The first traffic light was a railway-style semaphore.
3. Step 3. Traffic light — has part — LED. LEDs.
4. Step 4. LED — is part of — Torch (flashlight). Modern torches use LEDs.
5. Step 5. Torch (flashlight) — has part — Dry cell. Batteries.
6. Step 6. Dry cell — evolved from — Voltaic pile. A convenient form of the galvanic cell.

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