Prods inject current into the surface at two points, so the current spreads through the material between them and sets up a circular field looping around each current path. The field is strongest close to each prod tip and weakest midway between them, which is why the technique is qualified by prod spacing rather than by part size: the spacing fixes both the length of the current path and the depth to which the current spreads.

Because the field falls off away from the current path, a wider spacing needs proportionally more current, and the codes state the requirement as amperes per inch of prod spacing. Thicker sections let the current spread deeper below the surface, diluting the surface field, so they call for a higher figure: 90 to 110 A/in below 19 mm (3/4 in.) of thickness, and 100 to 125 A/in at or above it.

Spacing is bounded at both ends. Below 50 mm (2 in.) the field around each prod is so intense that the background of particles masks real indications, and the prods themselves get in the way. Above 200 mm (8 in.) the current has spread too widely to give a usable field at the midpoint, and the required current becomes impractical. The reliably examined area is the region between the prods, so coverage is built up from overlapping prod positions, repeated at right angles to catch discontinuities in both directions.

Prods pass current through the workpiece at a point contact, and that is the risk. Arc strikes and local burning cause metallurgical damage that is a defect in its own right on pressure equipment. Tips must be clean, contact firm before the current is switched on, and the current switched off before the prods are lifted. Many codes prohibit prods on finished or high-strength surfaces altogether.

Worked example

Material thickness	25 mm
Prod spacing	152.4 mm
Minimum current per inch	100 A/in
Maximum current per inch	125 A/in
Minimum magnetising current	600 A
Maximum magnetising current	750 A
Mid-band setting	675 A
Minimum permitted spacing	50 mm
Maximum permitted spacing	200 mm

A 25 mm thick plate, prods set 152.4 mm (6 in.) apart. The section is 19 mm or thicker, so the band is 100 to 125 A per inch of spacing. I_min = 100 x 6 = 600 A and I_max = 125 x 6 = 750 A, giving a mid-band setting of 675 A. The 152.4 mm spacing sits inside the 50 to 200 mm limits.