Passing current directly through a part sets up a circular field that closes on itself around the current path. Inside a solid conductor the field rises linearly from zero on the axis to a maximum at the surface, which is exactly where surface-breaking discontinuities are; and because the flux runs circumferentially, the technique reveals discontinuities lying longitudinally, parallel to the current. Nothing leaks at the part ends, so unlike a coil shot there is no demagnetising field to fight and no L/D ratio in the formula.

The surface field of a round conductor is H = I / (pi D), so the field per unit of current falls as the part gets bigger. Holding the field constant therefore means scaling current with diameter, and the codes express that as amperes per inch of diameter. Between 300 and 800 A/in covers everything from a light check on a machined surface to a demanding examination of a rough casting; 500 A/in is a common starting point, and values near the top of the band are used where the surface is rough or the expected discontinuities are tight.

For non-round sections, diameter means the greatest cross-sectional diagonal, since that is the dimension that sets the longest flux path. Large-diameter parts quickly demand currents beyond the machine, and that is the point at which the work moves to a central conductor, multiple contact positions or a prod technique.

Contact is the hazard. Every head shot carries burn and arc-strike risk if the contact area is small, dirty or the current is switched while the part is not clamped. Lead or copper braid pads are used on finished surfaces, and the current is always switched on and off with the part held firmly in the heads.

Worked example

Part diameter	50.8 mm
Current level	500
Magnetising current	1000 A
Code minimum current	600 A
Code maximum current	1600 A
Equivalent current density	19.69 A/mm of diameter
Surface field strength	6266 A/m

A 50.8 mm (2 in.) diameter shaft at 500 A/in. I = 500 x 2 = 1 000 A. The full code band for this part is 300 x 2 = 600 A up to 800 x 2 = 1 600 A. 500 A/in is 500/25.4 = 19.69 A per mm of diameter. Surface field H = I/(pi D) = 1 000/(pi x 0.0508) = 6 266 A/m, about 79 G. That is the ideal figure for a bare solid bar and it sits above the usual 30 to 60 G tangential acceptance band, so confirm the field with a Hall-effect probe on the actual part - a lower setting inside the 300 to 800 A/in range is often sufficient.