A coil device for use by divers in subjecting an elongate underwater strure member to a magnetic field for inspection purposes includes a framework supporting a winding having first and second coaxial coils of predetermined coil diameter that lie in substantially parallel planes separated axially by a distance at least twice the coil diameter, the coils being electrically connected in series so as to generate magnetic fields of opposite polarity. The coils, connecting conductors, and framework are adapted to be spread open and closed around the member to be inspected.