Definition
In magnetic particle inspection, the magnetic field produced lengthwise (along the long axis) of a part by passing electric current through a coil that surrounds the part, or by placing the part inside a solenoid. The resulting magnetic lines of force run parallel to the part's long axis, making the inspection most sensitive to cracks or discontinuities oriented across the part (transverse to the long axis).
Plain English
A way of magnetizing a part end-to-end so that hidden cracks running across it (sideways) show up during inspection.
Context Anchor
Seen in aircraft maintenance, especially magnetic particle inspection of steel parts such as bolts, shafts, landing gear parts, and engine components.
Derivation
From Latin longitudo, meaning 'length.' The field runs along the length of the part, which is why it's called longitudinal.
Why Pilots Care
Can create deviation errors in the magnetic compass that must be corrected before flight.
Analogy
Think of sliding a steel bolt inside a wire coil and energizing it: the bolt becomes a bar magnet with poles at each end, and the magnetic lines run from one end to the other along its length.
Intuition Check
Longitudinal does not mean stronger or more complete. It tells you the direction of the magnetism: along the length of the part.
Example Sentence 1
The technician used longitudinal magnetization to inspect the bolt for transverse cracks before reinstalling it in the engine.
Example Sentence 2
Longitudinal magnetization along the aircraft axis required degaussing before the compass could be swung accurately.