Why do Transformers Hum?

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Transformer hum is caused by a phenomenon known as magnetostriction. To understand why, it is necessary to take a look at how transformers work.§0 §0 Inside they contain two coils of wire, the primary and the secondary coils, wound onto opposite

sides of a ring made out of many thin sheets of iron or some other ferromagnetic material.§0 §0 An alternating current flowing through the primary coil generates an alternating magnetic field in the iron ring, which in turn creates a voltage in the

secondary coil. The ratio of the primary voltage to the secondary voltage is equal to the ratio of the the number of turns of wire in the primary coil to the number of turns in the secondary. This allows us to change the hundreds of thousands of volts

running through overhead power lines to a voltage low enough to be safe to use in our homes.§0 §0 The iron making up the ring that joins the primary and secondary coils is divided into microscopic domains. In each of these domains, the magnetic

field points haphazardly in different directions, much like a classroom full of unruly pupils who are running all over the place.§0 §0 However, when the iron is placed in an external magnetic field, these domains tend to line up and add together,

producing a strong magnetic field pointing in one direction, just as schoolchildren will snap to attention at a teacher's command.§0 §0 As the domains line up, the material very slightly changes its length to accomodate the rearrangement. This is

magnetostriction. As the magnetic field through the iron alternates, the iron expands and contracts over and over again. These vibrations produce the sound waves that create the transformer's distinctive hum.

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