Electromagnets, Transformers, Speakers,
Microphones and Pickups.

If a current is passed through a wire, a magnetic field is produced around the wire. If the wire is then wound into a coil and a current is passed through the coil, a magnetic field is produced.

This field is like a bar magnet with North at one end and South at the other. If the contacts are reversed then the current flows in the opposite direction and the magnetic field reverses. The strength of the magnetic field is proportional to the current times the number of turns on the coil, and how easy it is for the magnetic field to flow inside the coil. A core of iron will greatly increase the strength of the magnet as magnetism flows easily through iron.

When the current makes a magnetic field, this field itself tends to induce a current in the coil which is opposite to the original current. The ability to produce this reverse voltage is called its inductance symbol L. This is measured in Henrys.

It is the ability of a coil to create a magnetic field from a current and its converse ability to produce a current from a magnetic field that makes motors, microphones and dynamos work.

However it is not that simple, for though a DC voltage can produce a stable magnetic field, it requires that the magnetic field be changing for it to produce an induced voltage in the coil. The voltage produced by a changing magnetic field is proportional to the rate of change of the field i.e. the faster the field changes the higher the induced voltage.

The Guitar Pickup.

This consists of a coil wound around a bar magnet. It has pole pieces which, because they are made from iron, focus the magnetic field towards the string. Because the strings are made of steel (which contains iron) the vibration of the string changes the magnetic field. This field change induces a voltage change in the coils and this is the pickup output that you connect to your amp.

The pickup will also pick up stray electromagnetic interference from around you, like the mains hum from amplifiers and cables. You can explore the magnetic field of your environment by moving your guitar around the room and notice the hum changes levels. This hum and other unwanted sound induced in the pickups caused annoyance to guitarists so the humbucker was developed.Humbucker Pickup The idea behind it is easy. Use two coils, one wired the other way round. As the noise induced in each pickup is the same so connecting the the first coil up in phase causes that coil to produce + noise and the other coil being connected out of phase produces. These opposing signals cancel so that in theory the unwanted noise vanishes. The pickups are so designed that the signal is not cancelled. It does however have a different tone to the single pickup. The humbucker can also be used as 2 coils in series and in phase so it give nearly twice the output of a single pickup. You have the choice which you use. There was a fashion in the old days to modify the standard 2 single coil pick up arrangement by fitting a phase switch which enabled both pickups to be used in phase or out of phase.