Electric current always creates a magnetic field circling around it at 90° — the right-hand rule. Every MT technique is just a different way of aiming that field. A crack collects particles only when the flux runs across it (90° is best, and it still works down to about 45°). A crack lying along the flux is invisible — which is why every part gets magnetized in two directions before it passes.
An electromagnet shaped like a horseshoe. Current flows only in its winding — never in the part — and the part becomes the bridge between the two poles.
The flux runs pole to pole through the plate. Flaw ① lies across those flux lines, interrupts them, and collects particles. Flaw ② lies along them, so the flux slips past. To catch ②, rotate the yoke 90° and shoot again. No current ever enters the part.
The bar sits inside a coil of heavy cable. Current circles around the part in the windings — and the field it creates runs straight down the length of the bar.
Right-hand rule: current circles the bar, so the flux runs straight down its length. The transverse crack ② cuts across that flux and shows; the seam ① runs with it and hides. Now slide the coil left and right — the crack only indicates while the field still reaches it. On a long part you magnetize and inspect in overlapping steps.
The bench head-stocks clamp the part and pass current straight through it, end to end. This time the part itself is the conductor.
Exactly the opposite of the coil shot. Current runs through the bar, so the flux wraps around it in hoops. The longitudinal seam ① cuts across the hoops and shows; the transverse crack ② runs with the hoops and hides. Coil shot + head shot together cover both directions — that's the pair a bench inspection uses.
Two hand-held contact tips push current through a local patch of a big part — like a portable head shot for weld seams and castings too large for the bench.
The current crossing the plate is wrapped in circular flux, just like the head shot — so on the surface, the flux runs across the tip-to-tip line. Flaw ①, parallel to the prods, interrupts it and shows. Flaw ② hides; move the prods 90° to catch it. Real-world caution: prods pass current through the part — poor contact can leave arc burns.
For pipes, tubes and rings: a copper bar carries the current through the bore, and its circular field magnetizes the part around it — without a single amp touching the part.
Same physics as the head shot — circular flux around a current — but the current rides the copper bar, not the part. The pipe wall sits inside that circular field, so the longitudinal seam ① shows and the circumferential crack ② hides. Bonus: no arc-burn risk, and the inside surface is magnetized too — ID and OD in one shot.
| Technique | Where the current flows | Field direction | Finds cracks running… |
|---|---|---|---|
| Yoke | In the yoke winding — none in the part | Longitudinal, pole to pole | Across the pole-to-pole line |
| Coil shot | In the coil, circling the part | Longitudinal, along the part | Around the part (transverse) |
| Head shot | Through the part, end to end | Circular, wrapping the part | Along the part (longitudinal) |
| Prods | Through the plate, tip to tip | Circular, around the current path | Parallel to the prod line |
| Central conductor | Through the copper bar — none in the part | Circular, in the pipe wall | Along the pipe — ID and OD |