All useful electricity generators come in two basic forms, alternating current and direct current. Direct current (DC) comes from generators that do not change polarity, always producing a positive charge. In alternating current (AC), the polarity of the terminals always changes from positive to negative. So you end up with an alternating current flow. There are different ways to measure and generate AC and DC current. Direct current only flows in one direction in a circuit. Since the polarity of a DC voltage source is always the same, the current flow never changes direction. Batteries are one of the most common sources of DC voltage. Batteries are good because their voltage is fixed as well as their polarity. Direct current does not always need a constant voltage but it must always continue to flow in the same direction. There are such direct currents called variable and pulsating which change their value but not their direction. Alternating current always changes direction and amplitude. The AC current flow changes at regular intervals. Ac usually changes power and direction. The vast majority of electricity supplied to households and large businesses is alternating current. This is due to the ease of generating alternating current in alternators. The main concept of alternators is to move a conductor through magnetic lines. Changing the magnetic field around the conductor or vice versa causes the electrons to move. When you have physically moved the conductor or magnet in a complete 306 degree circle, you have produced a sine wave or complete cycle. The time required to complete a cycle is called period. The frequency of an alternating sine wave is the number of cycles per second. Frequency is measured in hertz. One hertz equals one cycle per second. The frequency coming out of your electrical outlet at home is 60 Hz. The maximum value of a sine wave is the highest voltage level away from zero. Peak-to-peak voltage value refers to the difference between the positive peak value and the negative peak value. The rms or rms value of a sine wave is the actual amount you would use or measure using a multimeter. In three-phase alternating current, there are three different coils or conductors that produce three different sine waves..