Definition
A frame made of easily magnetized iron used inside a flux gate compass to gather and concentrate the Earth's magnetic field so it can be measured electrically. Because soft iron magnetizes quickly when exposed to a magnetic field and loses that magnetism almost immediately when the field is removed, it can sense the direction of the Earth's field without retaining its own magnetism.
Plain English
A specially shaped piece of iron inside the flux gate compass that pulls in the Earth's magnetic field so the instrument can detect which way is north. It only acts magnetic while the Earth's field is touching it, then lets go.
Context Anchor
Seen in diagrams and explanations of the flux gate compass system, especially the remote compass sensor that supplies heading information to cockpit instruments.
Derivation
"Soft" here is a metallurgical term, not a description of physical softness. In magnetism, iron is called "soft" when it gives up its magnetism easily, and "hard" when it holds onto magnetism (like a permanent magnet). The frame is soft magnetically -- quick to magnetize, quick to demagnetize.
Why Pilots Care
The soft iron frame is the part of the flux gate compass that actually senses the Earth's field. Its behavior is what allows the system to deliver a stable, accurate magnetic heading to the pilot's instruments without the swinging and lag of a traditional magnetic compass.
Intuition Check
Do not read “soft” as weak, flexible, or low-strength metal. In this term, “soft” means the iron responds easily to magnetism and does not keep much magnetism afterward.
Example Sentence 1
The Earth's magnetic field flows through the soft iron frame of the flux gate compass, where sensing coils detect its direction.
Example Sentence 2
When the aircraft turns, the soft iron frame responds immediately to the new direction of the Earth's magnetic field.