Electrical currents normally flow in one direction through an electrical conductor. This kind of electricity, which is produced by batteries for example, is called direct current. When the electrical current is made to reverse its direction rapidly, it is known as alternating current. This kind of electrical current is used in domestic wiring, and it usually switches direction and back again about 50 to 60 times per second. This is the cause of the hum that you can sometimes hear near electrical equipment such as fluorescent lights.

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