maurits The innermost negative terminal on the socket is connected to the negative pole of your equipment's internal power source (usually batteries), and the outermost ("third") negative terminal completes the negative line to your equipment. When a plug from an outside power source is inserted into the plug, it disconnects the batteries, and supplies its own power to your equipment. (Of course the plug's positive terminal (the centre one) must be connected to the your equipment's positive line.)