The relay
How to drive an electric switch by electricity - The Relay.
The name "relay" (french: relais) was in use long before the spreading of electric technology: It was the common name for inns where tired horses could be exchanged to "fresh ones". The same problem occurred by electric telegraphy at long telegraph lines. The weak current after a long distance could still act a sensible relay to switch new current from a fresh battery.
Its principle is rather simple: An electro-magnet (that is a spool of insulated copper wire around a iron core) becomes magnetic, when a current is flowing between the points E1 and E2. When the current is off, the core will immediately loose its magnetism. Thus we got a magnet which can be switched on and off.
back