Yoke vs prods: what it means and why it matters

How each one works

A yoke is a magnet shaped like a U with adjustable legs. When the legs are placed on the part, the field travels from one leg to the other through the part, running along the line between the legs. No current enters the part, so there is no electrical contact to burn the surface. Yokes can be powered by AC, DC or both, or they can be permanent magnets. AC yokes are common because the changing field helps move the particles on the surface.

Prods are two hand-held electrodes pressed to the part. Current flows through the part between the tips, and the field circles that current path. The magnetized region is the zone between and around the prods. The setup is quick and works on parts too large to move to a bench, such as welds in the field.

Which flaws each shows

A yoke makes what is effectively a local longitudinal field, so it best shows flaws that run across the line between the legs. Prods make a local circular field, so they best show flaws running along the line between the tips. In both cases a single placement checks one direction, so the procedure normally calls for a second placement at about a right angle to cover flaws in the other direction.

Safety and condition

• Prods can arc. If the contact is poor, current can jump from the prod to the part and burn or harden a small spot, and for some materials that can create a new flaw. Surfaces must be clean at the contact points, prods should be pressed firmly and the current switched on only after contact and off before lifting. Many procedures restrict or forbid prods on certain materials.
• Yokes need a check too. A yoke is verified by showing it can lift a test weight, or by another method the procedure names, to show its field is strong enough. The weight and spacing come from the procedure.
• Leg spacing matters. For a yoke, a wider spacing covers more area but gives a weaker field. For prods, spacing and current go together. The numbers come from the procedure.

Mix-ups to avoid

• Yoke is not circular. It is local and longitudinal, even though prods are local and circular.
• Prods do not carry the yoke's field. The field exists because of the current, not because of a magnet.
• Spacing is not the same for both. One rule cannot be carried across to the other.

How the exam uses it

Equipment, techniques and selection of the method of magnetization are the usual places. A question might describe a weld in the field and ask which portable method shows a transverse crack, or ask what the risk of prods is, or what gives a yoke its field. Draw the part, the current or leg positions, and the field arrows.

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

Sketch yoke and prod setups against the magnetic particle testing quiz, then use the MT field direction and technique guide for the full workflow. The field types they make are compared in longitudinal vs circular magnetization, and the particles that go with them are covered in wet vs dry magnetic particles. All terms are listed in the NDT terms glossary.

Know the term? Check the rest of the MT General topics

Take the free 10-question check. It covers the MT General exam, needs no signup and explains every answer, so you see which topic to study next. The course then adds timed practice exams and a study plan built from your exam date.

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.

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