Almost every electrical and ignition system on a modern aircraft relies on one of nature's most powerful relationships: the interaction between magnetism and electricity. Whether you are troubleshooting an alternator, inspecting a starter motor, or testing an ignition coil, you are working directly with the principles of magnetism and electromagnetism. For the Aviation Maintenance Technician (AMT) candidate, a thorough grasp of these principles is not merely an exam requirement — it is a practical foundation for safe, competent maintenance work.
This article walks through the core concepts of magnetism, the connection between moving charges and magnetic fields, and how those ideas translate into the generators, motors, transformers, and ignition components found throughout the aircraft fleet.
Fundamentals of Magnetism
A magnet is any object that produces a magnetic field — an invisible region of force that attracts certain metals and interacts with other magnets. All magnets have two poles: a north pole and a south pole. The fundamental rule is that like poles repel each other and unlike poles attract. This repulsion and attraction are caused by the alignment of tiny magnetic domains — microscopic regions inside ferromagnetic materials (iron, nickel, cobalt, and their alloys) where atomic magnetic moments line up in the same direction. In an unmagnetized piece of iron, these domains point in random directions and cancel each other out. When the material is magnetized, the domains align, producing a net external field.
Magnetic field lines, sometimes called flux lines, are a convenient way to visualize this field. By convention, flux lines exit the north pole of a magnet, travel through the surrounding space, and re-enter at the south pole, forming closed loops. The density of flux lines at any point represents the strength of the field there. The total quantity of magnetic flux is measured in webers (Wb), while flux density (flux per unit area) is measured in teslas (T). Inside a high-permeability material like soft iron, flux concentrates readily, which is why iron cores are used in transformers and electromagnets.
Types of Magnets
- Permanent magnets retain their magnetism without an external source. They are made from hard magnetic materials such as alnico (aluminum-nickel-cobalt) or rare-earth alloys. Aircraft magnetos use permanent magnets to generate the rotating field that induces high-tension spark voltage.
- Temporary magnets are soft ferromagnetic materials that are easily magnetized by an external field but lose most of their magnetism when the field is removed. Soft iron cores in electromagnets and relays behave this way.
- Electromagnets are produced by passing current through a coil. The magnetic field exists only as long as current flows, and its strength can be controlled — making electromagnets far more versatile than permanent magnets in practical electrical components.
Electromagnetism: Current Creates Magnetic Fields
In 1820, Hans Christian Ørsted discovered that a compass needle deflected when held near a current-carrying wire — proof that electric current produces a magnetic field. This discovery is the cornerstone of electromagnetism. The magnetic field around a straight conductor forms concentric circles in a plane perpendicular to the wire. The direction of those circles is determined by the right-hand rule for conventional current (or the left-hand rule if you are tracking electron flow): grasp the conductor with your right hand with the thumb pointing in the direction of conventional (positive) current flow, and your curled fingers point in the direction of the magnetic field.
When the conductor is wound into a coil — called a solenoid — the individual fields around each turn add together, producing a strong, uniform magnetic field through the center of the coil. Adding a ferromagnetic core inside the solenoid concentrates the flux dramatically, creating a powerful electromagnet. The strength of this electromagnet depends on three factors: the current magnitude, the number of turns in the coil, and the permeability of the core material. The product of current and turns is called magnetomotive force (MMF), measured in ampere-turns (A·t).
Electromagnetic Induction: Magnetic Fields Create Current
Michael Faraday's 1831 discovery completed the picture: just as current creates a magnetic field, a changing magnetic field induces a voltage (electromotive force, or EMF) in a nearby conductor. This is Faraday's Law of Electromagnetic Induction. The induced voltage is proportional to the rate of change of magnetic flux through a circuit. Three conditions can create that changing flux:
- Moving a conductor through a stationary magnetic field (generator principle).
- Moving a magnetic field past a stationary conductor (alternator principle).
- Changing the strength of the magnetic field while the conductor remains stationary (transformer principle).
Lenz's Law describes the direction of the induced current: the induced current always flows in a direction that opposes the change in flux that created it. This is a consequence of conservation of energy — you cannot get electrical energy from induction without doing mechanical or other work against the opposing magnetic force. In practical terms, Lenz's Law explains why generators require mechanical input power, and why increasing the load on a generator makes the rotor harder to turn.
Aircraft Applications
DC Generators and Alternators
Aircraft DC generators use the relative motion between a rotating armature (with conductors) and stationary field windings to produce alternating EMF internally. A commutator and brushes rectify this to a pulsating DC output. Alternators — now far more common on modern aircraft — rotate a magnetic field past stationary stator windings, then use diodes to convert the resulting AC to DC. In both cases, field current controls output voltage: increasing field current strengthens the magnetic field and raises the induced EMF, which is exactly how voltage regulators work.
Electric Motors and Starters
An electric motor is essentially a generator run in reverse. When current flows through a conductor sitting inside a magnetic field, the field exerts a force on that conductor — this is the motor effect, governed by the left-hand rule (for conventional current). In an aircraft starter motor, this force rotates the armature. The torque produced is proportional to both the field strength and the armature current, which is why aircraft starters draw extremely high current during initial engagement when rotor speed (and back-EMF) is low.
Transformers
A transformer uses mutual induction — the changing magnetic flux from an AC primary winding induces a voltage in a separate secondary winding on the same core. The voltage ratio equals the turns ratio: if the secondary has twice as many turns as the primary, the output voltage is doubled (step-up transformer). Power (minus losses) is conserved, so a step-up in voltage means a step-down in current. Aircraft use transformers extensively in avionics power supplies and in transformer-rectifier units (TRUs) that convert 115 V AC bus power to 28 V DC.
Ignition Systems
The aircraft magneto is a masterpiece of applied electromagnetism. A permanent magnet rotor spins past laminated iron pole shoes wound with two coils — a primary and a secondary. As the rotating magnet's flux passes through the primary coil, it induces a current. At the precise moment of maximum flux change rate, breaker points open, collapsing the primary field almost instantaneously. This sudden collapse induces an extremely high voltage (often 15,000–20,000 V) in the secondary coil through transformer action. That pulse travels to the distributor and then to the correct spark plug — igniting the fuel-air mixture. The entire sequence relies on Faraday's Law operating at high speed with no external electrical source required, which is why a magneto-equipped engine can run with the aircraft battery completely disconnected.
Key Numbers and Rules
- Like poles repel; unlike poles attract — the first law of magnetic force.
- Magnetic flux is measured in webers (Wb); flux density in teslas (T).
- Induced EMF is proportional to the rate of change of flux, not the flux itself (Faraday's Law).
- Induced current opposes the change that caused it (Lenz's Law).
- MMF = current (A) × number of turns — increasing either strengthens the electromagnet.
- Transformer voltage ratio = turns ratio; power in ≈ power out (assuming ideal transformer).
- Aircraft magneto secondary voltages typically reach 15,000–20,000 V to fire spark plugs reliably across varying cylinder pressures.
- A magneto is self-contained — it does not require aircraft battery power to operate.
Common Test Traps
- Confusing flux with flux density: Total flux (webers) and flux density (teslas) are related but different. Test questions may ask which unit applies — density is flux per unit area.
- Lenz's Law direction errors: Students often forget that induced current opposes the change, not simply the original field. If flux is decreasing, induced current acts to sustain it — not to oppose the original field direction.
- Generator vs. motor rule: Both use conductors in magnetic fields, but the energy conversion is reversed. A generator converts mechanical energy to electrical; a motor converts electrical to mechanical. Test questions sometimes describe symptoms and ask which device is involved — focus on energy direction.
- Transformer operation requires AC: Transformers work only with alternating or pulsating current because a steady (DC) field produces no change in flux and therefore no induced secondary voltage. This is why the magneto primary circuit must be interrupted (by the breaker points) rather than simply energized with steady DC.
- Magneto independence from the aircraft bus: A common trap asks what happens to engine operation when the battery fails. Because magnetos are self-powered by their permanent magnets, the engine continues to run normally — only systems that rely on the battery or alternator are affected.