Wrapping the output cable of a portable unit several times around a part turns the cable into an encircling coil, producing a longitudinal field along the wrapped axis. It is the standard way to magnetise a shaft, a pipe section or a large weldment in the field, where no fixed coil bench exists. Because the turns lie directly on the part, the arrangement is a high fill factor coil: the cable is essentially in contact with the surface, so the flux does not have to spread across an oversized bore.

The field is set by ampere-turns, so wrapping more turns lets a small portable set reach the required magnetomotive force at a current it can actually deliver. Five turns at 1 000 A is the same 5 000 ampere-turns as one turn at 5 000 A, which no portable unit will supply. The demagnetising field from the part’s own end poles still governs how many ampere-turns are needed, hence the length-to-diameter ratio in the formula and its 2 to 15 validity band.

The usual field mistake is treating one wrap as covering the whole part. A coil’s field is only usefully strong for about 230 mm (9 in.) either side of the windings, so a long shaft must be wrapped, shot, unwrapped and re-wrapped further along with overlap. Turns should be kept tight against each other rather than spread out, since a spread wrap behaves like several weak coils instead of one strong one.

Cable heating is the practical limit. Wrapped cable cannot shed heat the way a straight run can, so keep shots short, respect the duty cycle of the set, and check the cable rating against the calculated current before starting.

Worked example

Part length	500 mm
Part diameter	100 mm
Cable turns	5
Cable diameter	20 mm
Cable current rating	1500 A
L/D used	5
Required ampere-turns	5000 A-turn
Cable current	1000 A
Current, lower tolerance	900 A
Current, upper tolerance	1100 A
Wrap positions along the part	2
Cable used per wrap	1885 mm

A 500 mm long, 100 mm diameter shaft with five tight turns of 20 mm cable. L/D = 500/100 = 5. Cable wrap is high fill factor, so NI = 35 000/(5+2) = 5 000 ampere-turns. Over 5 turns the set is run at 5 000/5 = 1 000 A, ±10% giving 900 to 1 100 A. Each wrap is effective 230 mm either side, i.e. 460 mm, so a 500 mm part needs 2 overlapping positions. Cable used = 5 x pi x (100 + 20) = 5 x 376.99 = 1 885 mm, plus leads.