On the Ringworld, the advanced City Builder machines were powered by an ubiquitous beamed power system. Energy was generated on the shadow squares by thermoelectricity, then beamed to the Ringworld. The Fall of Cities which ended the City Builder empire was caused by the failure of the beamed power system. [Spoiler alert: Ringworld Engineers] During the Second Ringworld Expedition it was discovered that a superconductor plague destroyed superconductor wires in the beamed power receivers.

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