While direct current (DC) refers to the flow of electrical charge carriers in a continuous direction, alternating current (or AC) refers to a periodic reversal of charge flow direction. As a mode of transferring electrical power, AC is tremendously useful because it allows us to use transformers to easily and efficiently step voltage up or down at will. If an electro-physical sensor senses a physical quantity that oscillates, the electric signal it produces will oscillate (AC) as well. For both these reasons, an instrument technician needs to be aware of how AC circuits work, and how to understand them mathematically.
