Most aircraft require some form of electrical power to operate navigation-, taxi-, landing-, strobe lights, one or more COM and NAV radio's, transponder, intercom and other advanced electronic system of your choice.
The electrical system consist of a battery and an alternator or generator on older aircraft. All of this is connected through several meters (kilometers in large aircraft) of wire.
All matter on Earth is made up from molecules and they basically consist of atoms. These atoms are made of electrons, protons and neutrons. And electricity is about the flow of free electrons attracted to protons and repelled by other electrons. Magnetism can and is used to generate electricity needed to operate lights, radio's and navigation equipment on-board of the aircraft.
We will take a quick look where and how these electrical systems use magnetism and how this energy can be stored for later use at our convenience.
This property is described by Aristotle to Thales in a discussion around 600 BC and it was also used in India by surgeon Sushruta Shamita for surgical purposes. In 1819 professor Hans Christian Oersted noted the relationship of electricity and magnetism as he discovered that an electrical current could influence a compass by running a current through a wire.
Magnetism enables certain materials (iron based) to attract (opposite poles) or repel (equal poles) each other. It is widely used in aircraft in applications as the compass, alternators/generators, starter motors, relays, navigation and a number of instruments.
The composition of magnetic material consist of minute particles each having a north and a south pole and they are all aligned in one direction. Non magnetized materials have exactly the same particles: the big difference is that these are not aligned at all but are layed out in a totally random, chaotic order.
In the area around a magnet exists a magnetic field and this can be proven by keeping a compass (which is also a small magnet aligning itself with the magnetic field of our planet) close to it. This magnetic field is called flux and it can be 'guided' by using soft iron. Soft iron has a permeability (this is comparable with electrical conductivity) which is some 1000 times greater than air, and it is this property that is used in electromagnetic relays, transformers and motors.
Some materials are magnetized without much difficulty, they also loose this property with the same ease. Soft iron is known for this ability. And this makes them very suitable for temporary magnets to be used in for example: relays, solenoid switches and other type of electromagnets.
Other materials are not so readily magnetized (hard iron, steel, neodymium (Nd2Fe14B)) but when they are magnetized they will not loose that property so easily. Which is the reason they can be used as a permanent magnet. You will need to hit them with a hammer or heat (up to the Curie point) them to make these loose their permanent magnetic field.
In an aircraft, you will find permanent magnets used as an iron detector in the lubrication system (Rotax four strokes), compasses or generators and as temporary magnets in alternators, electromagnets, relays, etc.
An electromagnet is basically an insulated wire wound a high number of turns around a core with a high permeability. If you send a current through that coil a magnetic field is created proportional to the amount turns in de winding and the current in the coil. Put differently: the strength of the magnetic field that is created depends on the current, the number of windings in the coil and permeability of the core that is used.
It works in reverse too: move a magnet through a coil and electricity is generated. This is the basic principle of the generator.
When the current through an electromagnet is switched off, a large reversed inductive voltage spike is created when the magnetic field collapses. The reason is that the coiled inductor resists changes in the current through its windings. And any change (switching off), will cause the energy stored in the coil to be returned to the electrical circuit with reversed polarity. Possibly with damaging results to the sensitive avionics.
To protect the electronics of the aircraft or any other electronic circuit with relays, this reverse voltage spike needs to be dampened. This can be done with a capacitor but most often a (Schottky) diode is used. A good example is the engine master / starter relay or any other type of coil which is switched on or off.