Abstract: There is disclosed a methodology for measuring the magnetic field strengths of fundamental and harmonic frequencies associated with a.c. electrical current flows over a protracted period of time; and, particularly, in hostile environments. A preferable application calls for the determination of the field strengths associated with electrical currents flowing in a.c. power, distribution lines of utilities. The monitoring instrument is positioned in relation to the power lines to be monitored, typically at the base of a pole immediately adjacent the pole of the power line or on a power line supporting pole itself. In order to provide for long term, powering of the monitoring device, d.c. power is developed utilizing the a.c. power that is present. The conversion from a.c. to d.c. is effected at a distance sufficiently far from the monitoring instrument so as not to have any consequence on the readings that are being taken.