Electromechanical relay
More info
- Type
- Electromechanical switch
- Released
- ≈1835
- Used in
- Telegraphy, relay computers
An electrically operated switch — a coil pulls an armature that opens or closes contacts — and the first technology fast and cheap enough to compute with.
A relay is a switch worked by electricity rather than a finger. Current through a coil magnetises a core, the core pulls an iron armature, and the armature snaps a set of contacts open or shut. It grew out of the telegraph in the 1830s — Joseph Henry demonstrated the principle around 1835 — where it "relayed" a weak signal into a fresh strong one, letting lines run for hundreds of miles.
Because contacts can stand for 0 and 1 and can switch one another, relays can do logic. Konrad Zuse built the Z3 from roughly 2,000 of them in 1941; Bell Labs and Harvard's Mark I did the same. Relays switch in milliseconds — a thousand times slower than a vacuum tube — so electronics soon displaced them for computing, but relay logic still runs lifts, railways and power stations where robustness beats speed.