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Electronic uses for magnetic materials

This is quite a long list, but it is probably not exhaustive – if you spot anything obvious that we haven’t included, please do get in touch! Apart from inductors and transformers, magnetic materials are also found in:

• Solenoids, used as actuators or as part of switches. These generally have a soft iron core, which has no residual magnetism. Current applied to a coil around a core energises the magnet, and moves actuator or switch.
• Relays are particular kinds of solenoid, designed for switching applications, and a conventional relay operates in the way described. However, a reed relay is a totally different beast. The coil in a reed relay provides a magnetic field, and the leaves in the switch are magnetic, so that the magnetism induced moves them relative to each other (generally to make contact).
• Safety mechanisms: the case in which you mount equipment may have a magnetic door catch and magnetic sensor as part of its tamper protection.
• Magnetic sensors are used for such purposes as revolution monitoring.
• Open your transistor radio, and you may well see a ferrite rod around which coils have been wound – in this case, the ferrite is acting as an aerial.
• For those with a historical bent, very small ferrite cores formed the basis of the original random access memory – 10ns was very fast in the 1960s, but manufacture and repair was a nightmare!
• Transmitting valves, such as klystrons and magnetrons use magnetic materials – think of that every time you use your microwave oven
• Many microwave components, such as circulators, depend on magnetic fields for switching signals.

Of course you might think that as your circuits don’t contain obvious magnetic materials, you don’t need to know anything about magnetism. Wrong! Most recording media are coated with fine particles of ferrite oxide or similar material orientated on a polymer base. The thin film has a ‘hard’ hysteresis loop, and the residual magnetisation of the film is proportional to the flux applied during recording. Recording heads themselves are made of ceramic ferrites or ferrites with thin films of metallic alloys within the recording head gap. You are able to read this material because it exists coded into the orientation of magnetic particles on a number of different computers.