Track circuit and heron’s aeolipile: what they have in common

1. Step 1. Track circuit — has part — Relay. Track relays.
2. Step 2. Relay — has part — Electromagnet. An electromagnet switches the contacts.
3. Step 3. Electromagnet — is part of — Pinball. Flippers are solenoids.
4. Step 4. Pinball — evolved into — Arcade machine. Coin-operated entertainment.
5. Step 5. Arcade machine — is a kind of — Vending machine. A coin-operated machine.
6. Step 6. Vending machine — was inspired by — Heron’s aeolipile. Heron described both a holy-water dispenser and the aeolipile; the dispenser is the prototype of the vending machine.

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