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The National MagLab is funded by the National Science Foundation and the State of Florida.

Ignition Coil

Start your engines and learn about the ignition coil, a key to operating your car.

Electric, solar-powered and alternate fuel vehicles are the wave of the future, but for now most cars run on gasoline, which they burn in an internal combustion engine to convert into motion.

For combustion to take place, a spark is needed to ignite the fuel mixture in the engine. The vehicle’s ignition system is designed so that a 12-volt battery can generate the very high voltage required to create such a discharge. The heart of this system is a device called an ignition coil.

Ignition Coil

This coil is a kind of transformer. Transformers transfer voltage from one circuit to another. The key principle that makes transformers work is electromagnetic induction: A moving magnetic field, or a change in a stationary magnetic field can induce a current in a wire exposed to that field.

This ignition coil consists of two coils of wire, as shown in the diagram. The secondary coil has far more turns of wire than the primary coil, which is wrapped around the secondary. This allows some 40,000 volts of electricity to be generated by a modest battery.


  1. Observe how the ignition coil is connected with a battery.
  2. Click the “charge coil” button, closing the knife switch to create a circuit, allowing current depicted by the yellow dots to flow to the coil.
  3. As current flows to the coil, you’ll notice the large magnetic field building up around it, depicted with the blue field lines.
  4. Press “ignite” to open the switch and stop the current. This causes the magnetic field to suddenly collapse, and induce a surge of current in the secondary coil, which streams out the high voltage output terminal and is enough to jump the spark gap.This spark then ignites the fuel mixture in your car’s engine and gets the motor running.
  5. Note the capacitor in the circuit. The collapsing field also induces a lesser surge (back EMF) in the primary coil that travels back through one of the primary terminals toward the switch. The capacitor is in place to safely absorb the back surge and keep it from creating a spark across the knife switch.