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Originally, electrical power was formed by chemical reaction, and that’s still the way that batteries work. This type of current, known as direct current (DC), flows from a negative pole, through an electrical device (such as a lightbulb), and on to the positive pole. However, direct current can’t be transmitted over long distances without a debilitating drop in voltage.
Utility companies now provide households with alternating (AC) current, which actually pulses—or reverses direction—120 times, or 60 cycles, per second (called 60 hertz power). AC power moves in waves, as shown below. Lightbulbs actually flicker as power ebbs and flows, but the human eye can’t detect it.
alternating current
(Note: An explanation is in order about the designation of the voltage supplied by the utility company. Voltage is electrical pressure. That pressure can fluctuate from roughly 115 volts to 125 volts, even within the same day, which is why you may see references elsewhere in this section to household voltages other than 120. This section uses 120 as the average voltage for each hot line from the utility company.)
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