Derailleur and electrical telegraph: what they have in common

1. Step 1. Derailleur — has part — Spring. A tension spring.
2. Step 2. Spring — is part of — Rotary dial. A return spring.
3. Step 3. Rotary dial — is part of — Telephone. A dial.
4. Step 4. Telephone — evolved from — Electrical telegraph. The telephone grew out of attempts to send several signals over telegraph lines (the “harmonic telegraph”).

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