Abstract: An electrically operated recording measuring instrument burns out the trace of a curve on a moving record medium coated on both sides with an evaporated metal layer. The power supply for the burnout current also feeds an auxiliary writing member for burning out an additional trace to isolate an area of the metal layer under the curve and, if desired, to subdivide the area in a similar way. The power supply also furnishes a higher voltage to a test probe for burning out any remaining conducting bridges across the burnout traces before capacitance measurements are made which can read directly in terms of area or relative area. The method and apparatus are particularly usable for photoelectrically evaluating blood samples treated by electrophoresis to separate the proteins.