In this scenario, we have a flat metallic surface featuring a diminutive aperture situated along its periphery. This metal top is positioned upon the film, and the technologist proceeds to rotate it swiftly while the exposure takes place. The objective of this particular examination is to assess the precision of the timer and ascertain the proper functionality of all four valve tubes. For instance, if we consider a fully rectified unit in an x-ray circuit with a total of 120 impulses, at a duration of 1/10 second, it is expected that 12 distinct dots will manifest on the film if both the timer and valve tubes are operating optimally.