In the 18th century, scientists still regarded electricity and magnetism as two entirely unrelated physical phenomena. Following the Danish physicist Ørsted's discovery of the magnetic effects of electric currents in 1820, the British physicist Faraday subsequently discovered the phenomenon of electromagnetic induction in 1831. These discoveries confirmed that electrical and magnetic energy could be mutually converted-a realization that laid the foundation for the subsequent invention of electric motors and generators. Consequently, through these inventions and innovations, humanity ushered in the Electrical Age. In the 1830s, while researching circuit control, the American physicist Joseph Henry utilized the principle of electromagnetic induction to invent the relay.
The earliest relays were electromagnetic relays; they harnessed the phenomenon of an electromagnet's magnetic force appearing and vanishing in response to the flow and interruption of current to control the opening and closing of a separate circuit-typically one involving high voltages and currents. The advent of the relay enabled the smooth execution of tasks such as remote circuit control and circuit protection. The relay stands as a monumental invention in the history of human technology, serving not only as a cornerstone of electrical engineering but also as a fundamental basis for the fields of electronics and microelectronics.
