As described just now, there is a flow of electric current when a load is connected between the two poles of a battery. Unfortunately some loads are sensitive to which way around they are connected (polarised) while others aren’t. Examples of non-polarised loads are conventional bulbs, relay and switch terminals and fuses. Practically any unit which includes semiconductors (transistors, PIC chips etc.) will be polarised, so make sure that you connect items such as speed controllers, sound units and receivers the right way around. They are usually marked with + (positive) and – (negative) signs/labels or at least the instructions will tell you how to connect them. Yes, it DOES matter! If you connect up a polarity-sensitive device the wrong way round, even for a split second, then you will probably damage it and often fatally. Those of us who have suffered this fate often then refer to 'The Magic Grey Smoke'……………!