Transformer hum is caused by a phenomenon known as magnetostriction. To understand why, it is necessary to take a look at how transformers work.

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.

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 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.

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.

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.

As the domains line up, the material very slightly changes its length to accommodate 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.

In the US, the mains voltage alternates 60 times every second (60 hertz), so the material expands and contracts 120 times per second, producing notes at 120 Hz and its harmonics. In Europe, where the mains supply is 50 Hz, the hum is nearer 100 Hz and its harmonics.

In addition to magnetorestriction there are two other reasons why transformers tend to emit sound.

The first is imperfect insulation. Just as the corona discharge from power lines in damp air produces a buzzing sound, insulation breakdown in a transformer can also be noisy. In practice, however, insulation breakdown usually occurs deep inside a transformer, where the heat stress is most severe, and no audible noise emerges until the final catastrophic failure.

The second is caused by moving parts. Power supplies such as those you find behind computers sometimes make a buzzing sound, which is most likely to be the wire winding moving as the transformer's magnetic field and the current passing through it act together to produce a force similar to that in an electric motor. On the face of it, it seems that eventually metal fatigue ought to set in, but in practice transformers seem to be able to keep buzzing for years.

Other parts of a transformer can also buzz. For example, if the clamps that hold the parts together are not fixed tightly, they can rattle inside the casing.

The description "silent servant" did not really mean that electricity was silent. The expression was coined in the early 1920s by the General Electric Company in the US and used in advertisements and popular magazine articles to promote the use of electrical equipment in homes. The idea it was meant to convey was that electricity, unlike humans, could perform tasks without speaking or being spoken to, not that electricity itself was silent. Indeed many pieces of electrical apparatus were, and still are, quite noisy.

Silent transformers do now exist and have been around since the early 1980s. The first models were too heavy and bulky for many types of equipment. But in modern appliances, "switch-mode" power supplies are used which have much smaller transformers supplied by alternating current at a frequency too high for humans to hear, with sharp-edged pulses rather than smooth signals.

The mains AC frequency of 50 or 60 hertz ­ which is noisy when passed through a transformer ­ is increased to the higher frequency, usually via an oscillator. The current then enters one or more transformers that step down the voltage and, thanks to the inaudible nature of the higher frequency, allow the transformer to perform quietly.

The more rapidly changing magnetic field allows smaller transformers to be used. So as well as converting the audible humming sounds to inaudible, ultrasonic whistles, it helps make equipment smaller and lighter.