Abstract: A line choke is disclosed for use in attenuating interfering emissions on a power line. The choke includes a ferrite core having four holes extending through it. A pair of windings are threaded through the holes and around the center of the core so that the windings crisscross to enhance the mutual coupling between them. Interfering emissions received by the choke are greatly attenuated at relatively low frequencies by the choke acting as a pi section filter, and higher frequency emissions are attenuated by the choke action as a very lossy transmission line.

Abstract: Levelling systems (e.g., scrapers) are laser-controlled by use of a sending station emitting a rotating, pulsed, laser beam in a plane either horizontal or inclined relative to the horizontal and a receiving station at the scraper which has a mast carrying one or more sensors. The sensor signals either signal the operator to raise, lower or change the angle of the blade or control the blade by hydraulic or electric actuations. The same mast mounted on a scraper (with blade raised to inoperative position) or other vehicle may be used for surveying a field. It is necessary in either case to raise and lower the mast so that the sensor intercepts the laser signal. This has heretofore been done in various ways hydraulically, electrically or mechanically (e.g., worm drive).