The relay let a feeble telegraph pulse travel as far as anyone wanted
Telegraph signals faded over long wires until someone had a clever idea: let the weak incoming current flip a switch that sends a fresh, strong copy down the next stretch. That switch, the relay, later ran telephone exchanges and early computers, and a descendant clicks inside countless machines today.
A relay is simply a switch operated by electricity. A small control signal on one pair of terminals opens or closes contacts on a separate circuit, so a weak current can command a powerful one, or one signal can drive several circuits at once. The contacts can be arranged to connect, disconnect, or both.
In the classic version, a coil wrapped around a soft iron core becomes an electromagnet when current flows. It pulls a hinged iron armature, which moves the contacts, while an iron yoke gives the magnetic flux an easy path. A spring holds the armature away when the coil is off, leaving an air gap. Cut the current and a restoring force roughly half the strength of the magnetic pull snaps it back, usually via a spring, though industrial motor starters often rely on gravity. Relays are built to act fast, which cuts noise in low-voltage uses and arcing at high currents.
Credit for inventing it is tangled. Samuel Thomas von Sömmerring designed an electrolytic relay as a telegraph alarm in 1809. Joseph Henry is often said to have made one in 1835, but he never published, and the dates rest on recollections decades later. In March 1837 Edward Davy lodged a letter with the Society of Arts describing a make-and-break electromagnetic relay, more practical than mercury-based designs, beating the first Wheatstone and Cooke telegraph patent by about eight weeks. Still, the first official patent went to Samuel Morse in 1840; his mechanism amplified and repeated the signal so it could be sent any distance. The word relay itself shows up in electromagnetic contexts from 1860.
Variations followed. Latching relays flip with a single pulse and stay put until a second pulse or reversed polarity resets them, useful where a power cut must not change the circuit. Solid-state relays swap moving parts for semiconductors. Protective relays guard against overloads and faults, and in modern power grids digital instruments doing that job still carry the old name.
Source: Relay
