I’ve always wondered how the relay got its name; I thought it was a strange moniker for an electromechanical switch. Reading The Information the other day, I finally found out. I had thought relays were originally invented to enable automatic circuit switching in telephone networks, but it turns out the telegraph was the necessity of their invention.
Telegraph signals weaken as they travel down a wire; beyond a certain distance the original signal can no longer power a receiver. So what do you do? You say, well, let’s have an intermediate telegraph operator: he can be stationed where the signal is still intelligible, and simply re-transmit what he receives, using his own power supply to generate a full-strength signal.
And then someone clever realizes you can use an electromagnet to operate the re-transmitting key and the incoming line to power it. Yes, the first relay was a telegraph key with an electromagnet stuck on it.