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're left with a flow of alternating current. There are several ways to measure and generate alternating and direct current. Direct current flows only 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 direct current voltage. Batteries are good because their voltage is fixed as is their polarity. The direct current dose does not always need a constant voltage but must always travel in the same direction. There are direct currents called variables and pulsators that change value but not direction. Alternating current always changes in direction and amplitude. The current flow in alternating current changes at regular intervals. Ac usually changes in strength and direction. The vast majority of energy 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 the movement of a conductor through magnetic lines. Changing the magnetic field around the conductor or vice versa causes the electrons to move. When you physically move the conductor or magnet in a full 306 degree circle, you have produced a sine wave, or a full cycle. The amount of time it takes to complete a cycle is called a period. The frequency of an alternating sine wave is the amount of cycles per second. Frequency is measured in hertz. One hertz equals one cycle per second. The frequency that comes out of your home electrical outlet is 60 Hz. The peak value in a sine wave is the highest voltage level away from zero. The value of the peak-to-peak voltage refers to the difference between the value of the positive peaks and the value of the negative peak. The effective or effective value of a sine wave is the actual quantity 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.