Whereas core-type current transformers have magnetic cores, flexible ones like Rogowski coils don’t have any. A Rogowski coil’s shell only houses a helical coil wire whose ends are located at the attachable terminals. If a Rogowski coil had a magnet inside, it would lose its flexibility due to the solid slab of magnetic material.
Instead, it relies on the electromagnetic field the conductor itself generates, which simply refers to the metal conduit. Flexible transformers rely on Ampere’s law to pick up the electromagnetic field looping around the wire. According to the law, current movement along this imaginary loop is the same as current movement along the conduit.
A Rogowski coil, therefore, must be manufactured with utmost precision. Since it doesn’t emit its own magnetic field, protection from electromagnetic interference rests on the proper spacing between the helical windings. On top of that, the closing point must connect both ends of the coil for accurate measurement.
Save for the substandard response to direct current circuits, however, flexible transformers are topnotch in every other aspect. Their lightness and flexibility enable on-the-spot conductor fittings without the need for invasive modifications to the electrical wiring. Furthermore, since it’s only a helical coil wrapped in insulating material, a Rogowski coil is affordable.