Definition
The number of degrees of cam rotation during which the breaker points of a magneto or ignition system remain closed, allowing current to build up in the primary winding of the ignition coil before the points open and produce the spark.
Plain English
The portion of the cam's turn during which the ignition contacts stay closed so the coil can charge up before firing the spark plug.
Context Anchor
Seen in aircraft magneto inspection, ignition timing, and breaker point adjustment.
Derivation
Dwell' comes from the Old English dwellan, meaning 'to linger' or 'to stay.' In this context it describes how long the points 'stay' closed during each rotation of the cam.
Why Pilots Care
Correct cam dwell produces a reliable spark; too little or too much reduces ignition energy, causing rough running, misfires, or hard starting.
Intuition Check
Dwell does not mean a delay added on purpose. Here it means the part of the cam’s rotation where the breaker points simply remain closed.
Example Sentence 1
The mechanic measured cam dwell during the magneto inspection to confirm the breaker points were adjusted within service limits.
Example Sentence 2
Adjusting cam dwell improved spark strength and smoothed the engine idle.