An improved Rogowski coil is formed on a toroidal core made of a thermoplastic or other moldable material, the core having a preferably continuous groove or grooves extending around the core. The grooves correspond in size to magnet wire which registers within the grooves, thus controlling the specific location of the wires. The grooving may be helical. A return loop can be provided for return path cancellation, or a reverse winding can be added in a direction opposite to the direction of advancement of the main coil. In using the return loop, a resistive network can be added to improve the cancellation of the return path due to the effect of geometries. In addition, it can compensate for thermal and other variations.