In 1835, shortly after the American Joseph Henry (1797-1878) became a professor at Princeton University, he passed crude on/off messages from his laboratory to his nearby home on campus using an electromagnetic relay and a current-carrying wire.
In the early relays, switching an electrical current on or off magnetized the relay's electromagnet core, and this magnetic field attracted a pivoting iron armature, which itself operated a set of contacts that made or broke an electrical circuit. One of the advantages of such relays was that the current through the electromagnet could be very small, and the switched-circuit current could be much larger.
Relays formed the basis of the telegraph system and were at the heart of telephone exchanges. In modern industry, electromagnetic relays are used to control electric motors and are a vital component of automated systems of machine manufacture.
Great advances have been made in relay design, mainly to guard against the potential damage caused by the voltage increase that is produced by the collapsing magnetic field of the coil when it is deactivated. Initially this was done by placing a capacitance and resistor in series, but in modern relays a simple diode is wired across the coil.
The power consumption of relays has been greatly reduced by introducing latching relays. These only consume power when they are switched on. By using a ratchet and cam system, these relays use one pulse to turn themselves on and a subsequent pulse to turn themselves off. Also, much attention has been paid to the relay contacts. Some are placed in vacuum tubes to prevent corrosion. Others are wetted with mercury to improve switching speeds.
