Jon writes -

Why on earth build a relay computer when there are already computers over a hundred thousand times faster? Well, for two reasons: to prove that silicon is not magic and because I want to.

The simple relay adder was built using free NOS relays sometime in 2005, but my goal was to someday build a more elaborate one that could run programs but not fill up a room like the early computers. Harry Porter built an incredible 415-relay computer which can be seen at this page…

I never thought I’d fully understand how his computer worked, let alone build one. Finally in spring 2007, I gained more understanding of computer systems from a course in college then joked to a friend about using the x86 instruction set for a relay computer to boot up Windows, which would’ve taken several hundred years on a relay computer running on a 50Hz clock. And so the inspiration began…

Relay Computer Two – Link.