structural:expansion, structural:summarization
History

Electrical relays got their start in application to telegraphs. American scientist Joseph Henry is often cited to have invented a relay in 1835 in order to improve his version of the electrical telegraph, developed earlier in 1831.

However, an official patent was not issued until 1840 to Samuel Morse for his telegraph, which is now called a relay. The mechanism described acted as a digital amplifier, repeating the telegraph signal, and thus allowing signals to be propagated as far as desired.

The word relay appears in the context of electromagnetic operations from 1860 onwards.