When was the electromagnetic relay invented?

Although there were several precursors that could be considered relays, the invention of the electromagnetic relay is attributed to Joseph Henry in 1835. Henry’s original application – and in fact the primary application of early relays – was in the operation of the electrical telegraph. In the early days of the telegraph, the signal strength would weaken as it traveled long distances, making it difficult to transmit messages reliably over extended wire lengths. To overcome this limitation, relay stations were placed along telegraph lines. The electrical signal was received at intermediate stations where it controlled a contact which regenerated and boosted the strength of the signal. This process was repeated as the signal traveled the line, ensuring that it reached the destination with sufficient strength to be interpreted. While the applications and use cases of relays has expanded well beyond the telegraph, the basic principles of design and operation of relays is largely unchanged.