Click here to purchase the entire book in PDF format. Alternating vs. Direct Current So far we have been talking about a constant supply of voltage - one that doesn't change over time, such as a battery before it starts to run down. This is what is commonly know of as direct current or DC which is to say that there is no change in voltage over a period of time. This is not the kind of electricity found coming out of the sockets in your wall at home. The electricity supplied by the hydro company changes over short periods of time (it changes over long periods of time as well, but that's an entirely different story...) Every second, the voltage difference between the two terminals in your wall socket fluctuates between about -170 V and 170 V sixty times a second (if you live in North America, at least...). This brings up two important points to discuss. Firstly, the negative voltage... All a negative voltage means is that the electrons are flowing in a direction opposite to that being measured. There are more electrons in the tested point in the circuit than there are in the reference point, therefore more negative charge. If you think of this in terms of the two tanks of water - if we're sitting at the bottom of the empty tank, and we measure the relative pressure of the full one, its pressure will be more, and therefore positive relative to your reference. If you're at the bottom of the full tank and you measure the pressure at the bottom of the empty one, you'll find that it's less than your reference and therefore negative. (Two other analogies to completely confuse you... it's like describing someone by their height. It doesn't matter how tall or short someone is - if you say they're tall, it probably means that they're taller than you. Secondly, the idea that the voltage is fluctuating. When you plug your coffee maker into the wall, you'll notice that the plug has two terminals. One is a reference voltage which stays constant (normally called a ``cold'' wire in this case...) and one is the ``hot'' wire which changes in voltage realtive to the cold wire. The device in the coffee maker which is doing the work is connected with each of these two wires. When the voltage in the hot wire is positive in comparasion to the cold wire, the current flows from hot through the coffee maker to cold. One one-hundred and twentieth of a second later the hot wire is negative compared to the cold, the current flows from cold to hot. This is commonly known as alternating current or AC. So remember, alternating current means that both the voltage and the current are changing in time.