What role did iron chains play in Christopher Wren’s design for the dome of St. Paul’s Cathedral?

In the design of St. Paul’s Cathedral, Sir Christopher Wren utilized iron chains to constrain the middle dome, which serves as the cathedral's primary structural support.1

Wren’s innovative solution for the dome involved a series of three nested shells. The middle shell, constructed of brick in a nearly conical catenary form, was designed to bear the significant weight of the lantern positioned at the top. By incorporating iron chains, Wren was able to effectively manage the structural loads and maintain the stability of this middle dome. This middle structure also supports a triangularly braced timber framework that carries the exterior lead-sheet surfacing, while a separate, shallower interior dome serves as the ceiling for the space below.1

This approach allowed Wren to apply his desired internal and external aesthetic forms to a concealed, mathematically informed structural core, a method that has since become a standard architectural technique.1