A nicely retro build here by Simon Winder, a special-purpose square root calculator using nixie tubes and a rotary phone dial:
via John Lluch on anycpu, who is building a relay-powered computer and was able to improve his square root routine using info from a 1977 issue of HP Journal, thanks to Simon’s notes.
There’s also a link to the relay machines on display at the Deutsches Museum, somewhere I might yet get to one day.