5.1 Magnetization by Electric Current: Contact, Prods, Central Conductor

ASNT NDT Level II magnetic particle general exams administered prior to 15 December 2026 still list Magnetization by Means of Electric Current as official topic 4. The written items are not asking you to name a bench brand. They are asking whether you can put a circular magnetic field in a ferromagnetic part, whether you know which hardware puts current through the part versus through a conductor, and whether you can apply a commonly taught amperage starting point without pretending that starting point outranks the governing procedure.

Circular field from electric current

A magnetic field forms concentric circles around a current-carrying path. In magnetic particle testing (MT) that family of flux is the circular field. The classroom picture is the right-hand rule: if the thumb of the right hand points in the conventional current direction, the fingers curl in the field direction.

A circular field is strongest for discontinuities that cut across the current path — longitudinal cracks on a bar, axial seams on a shaft, and axial cracks on a tube outside or inside diameter. It is weak for discontinuities that run parallel to the current. That is why a complete written practice almost always requires a circular shot and a longitudinal shot. This section is only the current-produced circular half. Coils, cable wraps, and yokes belong to the next section.

Two current paths produce a circular field in the part:

Current through the part magnetizes the volume the current actually occupies. Current through a separate conductor magnetizes the part from the field around that conductor. Hollow parts, coated parts, threaded bores, and burn-sensitive finishes change which path is legal. Exam stems that say "no electrical contact" or "do not damage the ID threads" are steering you off prods and often off headstocks.

Industry technique rules for these shots live in ASTM E1444/E1444M (aerospace practice), ASTM E709 (guide), and ASME Section V, Article 7, plus the employer written practice. ASNT does not publish one universal amperage table on the Level II outline. If a stem gives a procedure table, use the table.

Direct-contact head shot

On a wet-bench horizontal unit, the part is clamped between copper-pad or lead-pad headstocks. The operator selects a circular shot and the unit drives current through the part. Shop language is a head shot or direct-contact shot.

Typical use:

• Solid bars, shafts, pins, bolts, and other parts that fit the bench opening
• Production lots that need a repeatable circular field plus a later coil shot in the same fixture
• Wet fluorescent baths where the part can be flowed or immersed during the shot (continuous method)

The field is circular around the part's long axis when current is axial. Longitudinal surface cracks leak flux and collect particles. Circumferential cracks — a groove-like fatigue crack around a fillet, a heat-treat crack around a diameter — need a longitudinal field. Another head shot will not invent that field.

Pads must be clean, seated, and free of paint, heavy scale, and dried bath solids. Poor contact is how a "correct" amperage still fails to magnetize the part — or worse, arcs at the pad and pits a finished journal. Do not use the headstocks as a switch on a loosely held part. Clamp fully, then fire the shot.

Amperage starting point for through-part or encircling current

ASTM E1444/E1444M is explicit here: when magnetizing by passing current directly through the part, the nominal current shall be 300 to 800 A per inch of part diameter (12 to 32 A/mm). The diameter is the greatest distance between any two points on the outside circumference of the part — not the length, and not the wall thickness.

Inside that band the standard adds working guidance you should carry into the exam:

• Currents will normally be 500 A/in (20 A/mm) or lower. That is the practical starting point.
• The high end, up to 800 A/in (32 A/mm), is used to examine for inclusions or low-permeability alloys such as precipitation-hardened steels.
• Below 300 A/in is allowed only when part configuration dictates and approval is obtained from the Level III and the cognizant engineering organization.

That card is for through-part (head-shot) or central-conductor current, not for prods. The governing procedure or code table still wins. Confirm with the written practice and with a field indicator or quantitative quality indicator (QQI).

Examples at the common 500 A/in working point (full 300-800 A/in band in parentheses):

• A 2-inch-OD carbon-steel pin: about 1000 A (600-1600 A)
• A 2.5-inch pin: about 1250 A (750-2000 A)
• A 4-inch bar: about 2000 A (1200-3200 A)

Those are genuinely large currents, which is why a wet bench is a heavy, low-voltage, high-amperage machine. Increase or decrease only as the procedure and a QQI or pie-field indicator require. Hollow parts, non-round sections, and coated parts are not automatic diameter-card problems — the procedure will give an effective diameter or a different technique (often a central conductor).

Prod technique

Prods are handheld contact electrodes. Current still flows through the part, so the field is still circular — but it is local, between the two prods, not a full-length head shot. Prods are the usual field-portable circular method on large weldments, castings, and structures that will not fit a bench.

Common procedure limits taught in Level II courses — always confirm the written practice; these are not an ASNT-published single table:

Do not import the head-shot diameter card onto prods. Prod current is set per inch of prod spacing, and those numbers are far smaller than the 300–800 A per inch of diameter card. A 5-inch prod spacing on 1/2-inch plate is about 450–575 A total (90–115 A/in); the same spacing on 1-inch plate is about 500–625 A (100–125 A/in). Feeding a head-shot number such as 500 A/in of spacing into a prod shot would demand roughly 2,500 A through two small contact points — a burn-and-overheat setup, not a sensitivity gain.

The effective width of the magnetizing field is one fourth of the prod spacing on each side of the line through the prod centers. An 8-inch spacing therefore covers only about 2 inches each side of that centerline, which is exactly why placements must overlap instead of leapfrogging down a long weld.

Prod circular fields are strongest for discontinuities roughly perpendicular to the line between prods. Overlap successive placements and rotate the prod pair about 90° so you do not miss a crack that is parallel to the first current path. That two-direction rule is the same logic as a circular-plus-longitudinal bench exam, applied locally.

Prods are a poor first choice on finish-machined aerospace surfaces, thin sheet, carburized or nitrided skins that cannot tolerate a pit, and any procedure that forbids electrical contact. Those jobs move to a yoke or a coil.

Arc burn and overheating

Arc burn is a localized melt or pit from a spark at the contact. Many aerospace and pressure-equipment standards treat it as a rejectable surface condition, and the pit can start a crack. Classic causes:

• Prods set down or lifted while current is on
• Dirty, painted, scaled, rusty, or oily contact
• Loose hand pressure or a prod used as a switch
• Using the part edge as a knife switch against a pad

Overheating is a broader thermal stain, blueing, or temper color from too much current, too little spacing, or too long a shot. Both are technique failures. On the exam, "the Level II dragged live prods along the weld toe to save time" is an arc-burn stem, not a sensitivity improvement.

Central conductor

A central conductor — a copper bar, threaded rod, or internal cable — carries the current through the opening of a hollow part. The part is not in the electrical circuit. The circular field exists in the wall of the tube, ring, coupling, gear, or fitting.

Choose a central conductor when:

• You must inspect the inside diameter as well as the outside diameter
• Contact pads would damage threads, coatings, splines, or finished bores
• The part is a short ring or nut that would not get a uniform through-part current between headstocks
• You need circular magnetization of a thin wall without putting high current into that wall

The field intensity in the wall depends on the current in the conductor and the geometry. A multiple-turn internal cable multiplies ampere-turns. If the inside diameter is large and the procedure allows, offset the conductor toward one wall, inspect, then rotate and repeat so the near-wall field is adequate all around. A conductor rattling loosely in a large bore can leave a dead zone on the far wall.

Do not confuse a central conductor with a coil. A coil or cable wrap produces a longitudinal field along the part axis. A central conductor produces a circular field around the bore. A stem that says "longitudinal cracks on the ID of a coupling" wants a central conductor, not more coil turns.

Safety: arcing, burning, and wet parts

Electric-current magnetization is a high-current, low-voltage circuit. The hazards are thermal, electrical-contact, and ignition hazards, not utility-scale high voltage.

Arcing and burning. Any break in a high-current contact can pit the part and flash the operator. Clamp fully before a head shot. Never close the circuit by touching a live prod to the work.

Wet parts and explosion or fire risk. Wet fluorescent benches put the part in a petroleum or water-bath vehicle. Current plus a flammable film, or current plus a closed wet cavity, is an ignition and pressure hazard when:

• Bath collects in a closed hollow and a head shot is fired through that cavity
• Leads or prods arc in a solvent-rich fog
• A wet part is prod-shot and the spark lands in pooled vehicle

Common shop controls: drain closed cavities before a contact shot, keep prod work out of pooled bath, maintain ventilation, and follow the equipment manual and the bath safety data sheet. Water-based baths reduce fire load but do not remove shock, burn, or steam-blast risk on a poor contact.

Personal protection. Insulated prod handles, dry gloves, eye protection against ultraviolet-A (fluorescent inspection) and against sparks, and no rings or watches across the headstocks.

Choosing among the three current methods

On the general exam, "no contact permitted," "ID of a hollow fitting," or "arc-burn forbidden" steers you off prods and often off headstocks. "Longitudinal crack on a solid bar at the bench" steers you toward a head shot. "300–800 A per inch of diameter" is the through-part / central-conductor card; "90–125 A per inch of prod spacing" is the prod card. Keep those cards separate, and let the procedure win.