Longitudinal vs circular magnetization: what it means and why it matters
Circular magnetization puts a field in rings around a current path, made by passing current through the part or a central conductor. Longitudinal magnetization puts the field along the part, made with a coil or a yoke. Each finds best the flaws that run across its field, so the choice follows the flaw direction.
How each field is made
Circular magnetization comes from current. Pass current through the part, as with a head shot between two contacts or with prods, or pass it through a conductor running through a hollow part. The current creates a magnetic field that circles the current path, at right angles to it. The part itself can be any shape. The name describes the field, not the part.
Longitudinal magnetization comes from putting the part in a field that runs along it. A coil wrapped around the part does this, as does an electromagnetic yoke placed across it. The field lines run through the part between the ends of the field, which then act as poles.
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Which flaws each finds
A flaw produces a strong leakage field when it lies across the magnetizing field, near a right angle. A flaw that runs parallel to the field barely disturbs it. So:
- Circular field: finds flaws that run along the length of the part, because the rings cross them.
- Longitudinal field: finds flaws that run across the part, because the field runs through them.
If flaws could lie in either direction, the procedure uses both fields, one after the other or together, so that every direction is covered. A part that is only magnetized once is only inspected for one direction. The related page on the leakage field explains why.
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Every answer comes with a full explanation.
Things to keep straight
- Current direction is not field direction. In circular magnetization the field is at right angles to the current. A flaw along the current direction is crossed by the rings.
- Coil length matters. A longitudinal field in a coil works well on parts that are long compared with their diameter. Short, stubby parts have strong end poles that can cause trouble, and the procedure may use extensions to adjust for that.
- Poles can cause false indications. The ends of a longitudinally magnetized part produce strong fields that can attract particles with no flaw present.
- Direction of the field drives coverage. No setting of current or time can make up for a field that lies along the flaw.
How the exam uses it
The topics on magnetization by electric current and on selecting the proper method of magnetization are built on this. Questions give a sketch of a part and a flaw direction and ask which technique or which field will show it. Draw the flaw and the field as separate arrows and check that they cross. The MT field direction and technique guide gives a workflow for it.
Topic names here follow the MT General outline for exams before 15 December 2026; the MT outline change guide shows how the later list lines up.
Practice it
Work the drawn setups in the magnetic particle testing quiz, then follow the sketch routine in the MT field direction and technique guide. Portable ways to make each field are compared in yoke vs prods, and why the flaw direction matters is in the leakage field page. See all terms in the NDT terms glossary.
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Sources
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Background reading: PCC's magnetic particle section notes and ASNT's published topic headings. Outline checked 25 October 2026.
- Magnetic particle testing section notes, Portland Community College.
- ASNT NDT Level II examinations, published topic headings.
Not affiliated with or endorsed by ASNT.
Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.
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