4.2 Flux Fields: Circular vs Longitudinal

The second official MT general topic on the outline administered prior to 15 December 2026 is Flux Fields. The exam cares about one practical result: a given magnetization produces field lines in a known direction, and only discontinuities that cut those lines leak.

Right-hand rules you must be able to draw

Two right-hand rules cover almost every magnetization sketch on the general paper.

Circular field around a current-carrying conductor. Point your right-hand thumb in the direction of conventional current (positive to negative). Your fingers curl in the direction of the circular magnetic field around that conductor. If the 'conductor' is the part itself — a head shot or other direct-contact magnetization — the field is circular inside the part, in planes perpendicular to the current.

Longitudinal field of a coil. Curl your right-hand fingers in the direction of current around the coil (or cable wrap). Your thumb points along the coil axis in the direction of the longitudinal field through the part.

A central conductor (copper bar or cable through a hollow part) is still a circular-field rule: current is in the bar, not in the part, but the field in the wall is circular around that bar. That is how you examine the inside diameter (ID) of a ring or coupling without passing current through the article. Field strength is highest near the conductor and weaker toward the outside diameter (OD), so a large-diameter ring may need more current or multiple-turn central conductors to meet the procedure at the OD.

An electromagnetic yoke is a portable U-shaped electromagnet. The field in the part between the legs is essentially longitudinal along the line joining the poles (a local horseshoe). Treat yoke inspection as longitudinal magnetization of the path between the feet. Rotate the yoke about 90° for the second direction.

Circular fields: what they find

Circular magnetization is produced by:

• Current through the part (headstocks, clamps, prods)
• Current through a central conductor
• A cable passed through a hole so current is along the hole axis (same circular geometry)

Flux lines go around the current, like rings on a barrel. On a shaft or bar, those rings are circumferential.

A discontinuity leaks when it cuts those rings. That means:

• Longitudinal discontinuities — seams, laps, stringers, and cracks that run along the length of the bar, parallel to the current — cut the circular flux and give strong indications.
• Circumferential or transverse cracks that wrap around the bar run with the circular flux and leak poorly.

Memory line the exam likes: a circular field detects discontinuities that are roughly parallel to the current and therefore longitudinal on the part.

Prod magnetization is a local circular field. Current enters one prod and leaves the other. Between the prods the current path is the line joining them, so the circular flux is around that line. Discontinuities that lie parallel to the line of the prods (parallel to the current) cut that flux. Prod technique, spacing, and arc-burn risk belong to the magnetization chapter; the Flux Fields point is direction only.

Longitudinal fields: what they find

Longitudinal magnetization is produced by:

• A rigid coil or a cable wrap around the part
• An electromagnetic yoke
• Sometimes an induced-current setup that still produces an axial field in the article

Flux runs along the long axis. Discontinuities that cut that axis — transverse cracks, circumferential cracks on a shaft, weld cracks that run across a pipe — leak well. A long seam running down the bar is almost parallel to the field and is a classic miss on a coil-only exam.

Memory line: a longitudinal field detects discontinuities roughly perpendicular to the long axis (transverse / circumferential).

Two directions about 90° apart

One field direction is never a complete examination. ASTM E1444 and ASME Section V, Article 7 require magnetization in at least two directions approximately perpendicular to each other, or a documented multidirectional (vector-field) technique that produces a swinging field covering those orientations. A 90° pair covers the worst-case crack that was parallel to the first field.

Typical shop pair on a shaft: circular head shot (finds longitudinal seams) plus coil shot (finds transverse cracks). Typical weld-toe pair: yoke placed so the field is across the toe, then the yoke rotated about 90°.

Do not invent a third official angle. 'About 90°' is the requirement. Two shots 45° apart are not a substitute for two perpendicular fields. A single 'strong' circular shot does not find a circumferential quench crack.

L/D ratio and coil fill factor

Coil shots fail quietly when the part is a stubby disk or when it rattles in an oversized coil.

Length-to-diameter ratio (L/D) is the part's magnetized length divided by its diameter (use an effective diameter for non-round sections). A low L/D part has poles too close together; much of the flux leaves the ends and never fills the mid-length. Common teaching, and the usual reading of ASTM E1444, is that L/D should be at least about 2 for a satisfactory longitudinal coil field. Some training notes prefer 3 or 4. If the part is too short, use pole pieces or ferromagnetic extenders to increase effective length, switch to a different magnetization, or accept that a coil shot is the wrong tool. The procedure states the minimum L/D it will accept.

Fill factor is the fraction of the coil's cross section occupied by the part. A part that nearly fills the coil is high fill; a bolt hanging in a 20-inch coil is low fill. Low fill couples poorly. You need more ampere-turns.

Two commonly taught coil formulas appear in training courses and are also printed in ASTM E1444 (they are industry-practice formulas, not unpublished ASNT exam secrets):

• Low fill-factor coil (part much smaller than the coil): NI = 45,000 / (L/D)
• High fill-factor coil (part nearly fills the coil): a common companion form is NI = 35,000 / (L/D + 2)

N is the number of turns, I is the current in amperes, NI is ampere-turns. If L/D = 5 and the coil is low-fill, the commonly taught low-fill formula gives NI = 45,000 / 5 = 9,000 ampere-turns. A 5-turn coil would then be set near 1,800 A — if that formula is the one the procedure adopted.

The procedure's formula governs. ASTM E1444 and ASME Section V, Article 7 publish magnetization formulas and tables that may use different constants, units, or fill-factor definitions. On the specific exam, if a procedure quotes a formula, use that formula. On the general exam, know what L/D and fill factor do, recognize 45,000 / (L/D) as the usual low-fill teaching formula, and never apply a coil formula to a yoke or a prod shot.

Residual versus continuous method

Field direction is set by how you magnetize. Field magnitude and sensitivity are set by whether you inspect in the active field or in remanence.

Continuous method: particles are applied while the magnetizing force is on. Use it when sensitivity matters — welds, in-service fatigue, low-retentivity steels, and essentially all AC yoke work. AC does not leave a useful residual field. On a wet bench the bath should be flowing (or freshly applied) while the current pulse is on.

Residual method: magnetize (usually DC, full-wave, or half-wave), then apply particles after current-off. Use it on high-retentivity parts in production (bolts, bearing races, many aerospace fittings) when the procedure has verified residual field. Faster, cleaner, less particle spray in the coil — and less sensitive than continuous on the same part.

A circular residual field after a head shot still finds longitudinal discontinuities; a longitudinal residual field after a coil shot still finds transverse ones. The direction rules do not change. Only the magnitude of B changes (remanence versus applied field).

Realistic exam scenarios

A 2-inch-diameter, 24-inch-long carbon-steel bar (L/D = 12) is head-shot circular, then coil-shot longitudinal. The circular shot is for seams along the bar; the coil shot is for quench cracks around the circumference. Skipping the coil shot because 'we already magnetized it' fails the 90° rule.

A 1-inch-long washer cannot be coil-shot by itself (L/D ≈ 1). Stack it with ferromagnetic extenders or use a central conductor for circular magnetization of the hole and a different technique for the faces.

A wet-bench operator applies fluorescent bath only after the AC coil current is off. That is an accidental residual technique on a current type that leaves almost no residual. Indications disappear. The fix is continuous application — bath flowing while current is on — not more amperage.

A hollow coupling is magnetized with a central conductor. The field in the wall is circular, so the exam is looking for discontinuities along the axis of the coupling (longitudinal on the wall), not for a circumferential crack that follows the circular flux. The circumferential crack needs a longitudinal field (coil or cable wrap around the coupling).

A procedure cites NI = 45,000 / (L/D) for a low-fill coil. The candidate must compute NI from the procedure numbers, not from memory of a different textbook constant.

What Flux Fields items are really testing

Name the current path, draw the field with a right-hand rule, then ask which way the crack must run to cut that field. Add L/D and fill factor when the stem is a coil. Add residual versus continuous when the stem turns the current off.