In this paper we propose a novel optical method for measuring the circular magnetic field. In practice, many situations may appear in which there are difficulties in measuring the magnetic field, as inside coils, motors etc., where the magnetic field lines are circular or elliptical. The proposed method, applied for measuring the current on high voltage lines, strongly benefits from the advantages that it offers as compared to classical solutions based on the inductive principle. Some of the advantages of optoelectronic and optic measurement methods have a real importance. These advantages consist in: avoiding the use of energy intensive materials (Cu, Fe etc.), reducing the weight of the measuring system, reducing at the minimum the fire danger due to the use of paper-oil insulation in high voltage devices etc. The novelty of our proposed method consists in using the electromagnetic radiation in ultrashort pulses, having a relatively large frequency band and a much improved resistance to external perturbations, for measuring the circular magnetic field generated from the current of high voltage lines, inside power transformers or high power motors.