Milling machine and optical telegraph: what they have in common

1. Step 1. Milling machine — evolved into — CNC machine. The first CNC machines were milling machines (MIT, 1952).
2. Step 2. CNC machine — has part — Servomotor. Servo drives move the axes.
3. Step 3. Servomotor — uses — Electromagnetism. A servo is an electric motor with feedback.
4. Step 4. Electromagnetism — is used in — Electrical telegraph. A telegraph receiver is an electromagnet that attracts an armature.
5. Step 5. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.

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