Abstract: This invention relates to a method and apparatus for determining the relative position between the point of impingement of an electron beam on a work surface and the seam between parts to be welded. The seam is scanned by oscillating a low power electron beam along a path transverse to the scan. The variation in secondary electrons generated as the beam traverses the seam is detected and amplified. The amplified signal is then acted upon by an electronic converter which converts the wave form by instantaneously taking the Nth root at each point of the curve representing the wave form of the amplified secondary electron current wave form so as to improve the resolution of the signal and make possible an improved definition at the point of impingement of the beam with respect to the seam.