MT field direction and technique selection

Draw the part, discontinuity and field separately

For a study sketch, give the part an axis and mark the expected discontinuity direction. Then add the field direction as a separate arrow. A field that crosses a discontinuity at right angles is the useful starting model for detection; a field running along it is a reason to reconsider the setup.

Keep three labels on the sketch: current path, field direction and discontinuity direction. They are different things. Saying only that a setup is circular or longitudinal leaves out the relationship that makes the choice useful.

Compare setups by what they do in the part

Setup to review	Field relationship to sketch	Question to resolve before choosing it
Head shot	Circular field around the current path through the part	Is the discontinuity orientation suited to that field, and are contact conditions acceptable?
Central conductor	Circular field around the conductor	Can the conductor arrangement give the required coverage in the hollow part?
Coil	Longitudinal field along the part	Does this field cross the discontinuities you are seeking?
Yoke	Field between the poles	How should the legs be placed relative to the expected discontinuity?
Prods	Field around the current path between contacts	Have you distinguished the contact line from the field direction?

Use this as a comparison sheet, not an operating procedure. Current, spacing, coverage, contact restrictions and verification come from the applicable written procedure and equipment instructions. A memorized current formula cannot establish those conditions by itself.

Work through one study sketch

Draw a bar with a discontinuity along its length. Compare a circular field with a longitudinal field on that same sketch. Explain which one crosses the discontinuity and which one tends to run along it. Then rotate the discontinuity across the bar and repeat the comparison.

The useful result is the reasoning on the sketch. Do not turn the sketch into a general instruction to inspect every bar with one setup. Real selection also depends on the part, access, material, equipment and controlled procedure. If orientation is uncertain, your study answer should identify how coverage in the required directions will be demonstrated rather than simply name one shot.

Finish the choice with evidence

Technique selection is incomplete until you can explain how the process is checked. Use the equipment and evaluation modules to ask:

• How will field direction and adequate coverage be verified?
• Which particle and viewing conditions does the procedure require?
• How will a relevant indication be distinguished from geometry or process effects?
• What record will preserve the observation and its evaluation basis?
• If demagnetization is required, how will the result be checked?

Leave numerical limits on the controlled procedure or the stated practice problem. Do not transfer an illustrative value from a calculation into a different job.

Make direction errors visible in your practice notes

For each missed technique question, redraw all three directions before reading the explanation again. If the sketch is correct but the answer is still wrong, look next at material, contact restrictions, coverage or process verification. The MT practice routine provides an error-log structure for that review.

For date-specific topic changes, read the MT outline guide. This workflow develops a study skill; it does not supply inspection acceptance criteria.

Sources

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Study basis: Angle Beam's MT modules on magnetism and fields, leakage fields and current, technique selection and demagnetization, and evaluation and process control. Outline checked 19 October 2026.

• ASNT NDT Level II examinations, published topic headings.

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