Make: Electronics by Charles Platt is one of my current favorite books. I’m working my way through it, experiment by experiment, and learning tons about circuits, components, soldering, schematics, and more along the way. Recently I was working with switches and relays and learned an interesting bit of etymology.
Switches permit the controlled connection, or disconnection, of a circuit. We’re all familiar with light switches, doorbells, computer on/off switches, etc. Less familiar (now) is the telephone switchboard, for which an operator had to be able to selectively connect together pairs of the thousands of possibilities, as quickly as possible.
Charles E. Scribner, a man of great ingenuity, developed the “jack-knife switch” to make fast switching possible. This was a plug with a jackknife-like handle (hence the name) that could be inserted into a socket to activate the switch. Two such plugs at either ends of a cord allowed the connection of any two sockets, yielding an active telephone connection.
Although switchboards are no longer used today, the same jack-knife switch design was used to design audio connectors — which is why we refer to those plugs as “jacks”, even though no knife handle remains. I had never thought to wonder why they were called that!