I suppose that this doesn't really belong in the 'Physics' section; it's a little too non-technical to fall under the category of a physics description. There's really no better place to put it, though, so I'll just put it here.
The ¼ inches jack works by having a series of metal springs push up against the side of the ¼ inches plug, making electrical contact with the different segments of the plug. This diagram shows the two metal springs that we use in this circuit; the width of the jack, relative to the width of the plug, is greatly exaggerated for clarity.
Diagram showing connections in a 1/4 inches audio jack.