A system is described for using uhf dipole resonance in uniaxially anisotropic permalloy films to detect and measure weak magnetic fields. The dipoles are pulled to a position at right angles to the easy axis where they approach a condition of infinite permeability. Dispersion of the anisotropy over the film causes a severe degradation of the permeability but is largely overcome by low level modulation of the field with a low frequency signal. The net movement of the dipoles in response to the weak field being tested is measured both in magnitude and direction using simple uhf dipole resonance, the detected output being phase detected in turn with the low frequency source acting as the reference. Experimental results, which closely approach the theoretically predicted results, have been obtained down to 10−9Wb/m2sensitivity in 1 cps bandwidth in ordinary laboratory environments and using a 35 dB noise figure receiver. Means for greatly improving both the sensitivity and bandwidth are discussed.