Definition
An electrical induction coil used in some piston-engine ignition systems to step up a low-voltage input (typically from a battery or magneto primary) to the high voltage required to produce a spark across the spark plug electrodes. It consists of a primary winding of relatively few turns and a secondary winding of many turns wound around a common iron core; when current in the primary is interrupted, the collapsing magnetic field induces a high-voltage pulse in the secondary.
Plain English
A small transformer-like device that takes a low voltage in and produces a very high voltage out, strong enough to jump a gap inside a spark plug and ignite the fuel-air mixture.
Context Anchor
You will see this term in piston-engine ignition system discussions, especially when studying how spark plugs are fired.
Derivation
From 'spark' (the visible electrical discharge) and 'coil' (wound wire). The name simply describes its job: a coil of wire that produces a spark. The 'how' lies in two coils sharing a magnetic core, which is what allows a small input voltage to be transformed into a much larger output voltage.
Why Pilots Care
A faulty spark coil can cause engine misfires, hard starting, or complete ignition failure, directly affecting flight safety and reliability.
Intuition Check
A spark coil does not store sparks. It creates a brief high-voltage electrical pulse that lets the spark plug fire.
Example Sentence 1
When the spark coil failed, the engine would not produce a spark at the plugs and refused to start.
Example Sentence 2
A weak spark coil caused the engine to run rough on one magneto during the preflight run-up.