Shaft and electrical telegraph: what they have in common

1. Step 1. Shaft — is part of — Universal joint. Connects two shafts.
2. Step 2. Universal joint — is part of — Car. A universal joint carries rotation to the rear axle.
3. Step 3. Car — has part — Turn signal. Turn signals.
4. Step 4. Turn signal — uses — Relay. A flasher relay.
5. Step 5. Relay — is part of — Electrical telegraph. Relays boosted the signal on long lines.

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