Incandescent gas mantle and telephone: what they have in common

1. Step 1. Incandescent gas mantle — is part of — Gas lamp. The Welsbach mantle glows in the flame (from the 1890s).
2. Step 2. Gas lamp — uses — Coal gas. Coal gas.
3. Step 3. Coal gas — is used in — Traffic light. A gas lamp lit the signal.
4. Step 4. Traffic light — has part — Relay. Relay controllers.
5. Step 5. Relay — is part of — Telephone exchange. Relay switching.
6. Step 6. Telephone exchange — is used in — Telephone. Connecting subscribers.

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