Electricity comes in two flavors: alternating current (AC) or direct current (DC). With direct current ,the stuff that comes out of your batteries, electricity flows from the negative end of the power source through a circuit, then back into the positive end. Shouldn’t it be the other way around? Probably, but the poor guy who chose which end to call positive and which to call negative didn’t know about electron flow at the time and…guessed wrong. With alternating current, the stuff that’ll kill you if you get in its way, the flow of electrons switches direction several times a second. To be precise, the frequency of the change in direction is 60 times a second in North America, and 50 times a second in much of the rest of the world. Which, incidentally, is why traditional NTSC video was 30 frames per second, give or take a drop frame, and PAL video was 25 frames per second. Both multiply twice into their countries’ respective AC frequencies. AC is how utilities companies get electricity from the power station to your home or office. The frequency is measured in Hertz, so the alternating current going to your building will either be 50 Hz or 60 HZ, depending on where you live.