Sliding middle scale and optical telegraph: what they have in common

1. Step 1. Sliding middle scale — is part of — Slide rule. A movable scale.
2. Step 2. Slide rule — is a kind of — Computing device. An analogue calculator.
3. Step 3. Computing device — includes — Turing’s Bombe. A computing device.
4. Step 4. Turing’s Bombe — has part — Relay. Electromechanics.
5. Step 5. Relay — is part of — Electrical telegraph. Relays boosted the signal on long lines.
6. Step 6. Electrical telegraph — evolved from — Optical telegraph. The electrical telegraph replaced semaphore lines.

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