3D printer and railway semaphore: what they have in common

1. Step 1. 3D printer — uses — Program control of a machine. The layers are set by a program (G-code), as with CNC.
2. Step 2. Program control of a machine — is used in — Zuse Z3. The program was on punched cinema film.
3. Step 3. Zuse Z3 — has part — Relay. 2,600 relays.
4. Step 4. Relay — is part of — Traffic light. Relay controllers.
5. Step 5. Traffic light — evolved from — Railway semaphore. The first traffic light was a railway-style semaphore.

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