Abstract: Apparatus for detecting surface defects in steel billets comprises an h.f. energized eddy current probe for scanning over the surface of the billet and a dual channel receiver responsive to signals from this probe dependent on (a) any surface defects, and (b) the probe surface spacing, in a manner such that in one channel a predominant factor in the signal waveform is characteristic of (a) while in the other channel a predominant factor in the signal is characteristic of (b). The signals from the two channels are combined in a summing amplifier and a difference amplifier whereby the outputs therefrom are conditioned to be exclusively characteristic of (a) and (b) respectively.