Churn dasher and hollerith tabulator: what they have in common

1. Step 1. Churn dasher — is part of — Butter churn. A dasher plunger churns the cream.
2. Step 2. Butter churn — uses — Lever. A dasher.
3. Step 3. Lever — is used in — Railway semaphore. The semaphore is set by a lever.
4. Step 4. Railway semaphore — evolved into — Traffic light. The first traffic light was a railway-style semaphore.
5. Step 5. Traffic light — has part — Relay. Relay controllers.
6. Step 6. Relay — is part of — Hollerith tabulator. Relays controlled the sorting box.

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