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Tube Face: A typical G-M tube face consists of a small tungsten wire (approximately 0.3 mm in diameter) acting as the anode (positive electrode) and located on the axis of a tube face. The tube face may be glass with a contained metal cylinder to act as the cathode (negative electrode) or the interior surface of the glass tube face may be covered with a thin layer of an electrical conductor so that the Wall itself acts as the cathode. Usually the tube face is filled with a gas (as argon) or mixture of gases and its pressure is reduced to from 2 to 10 centimeters of mercury. Geiger-Muller counters can be made for counting of any type of radiation particle or photon; alpha particles are the most difficult to isolate, particularly as they necessitate a very thin window in the chamber in order to allow penetration.
The largest single project in the system was the $180 million Trans-Bay tube face that extends 3.6 mi (5.8 km) under San Francisco Bay. When underground approaches are included, the total length of the tube face will be about 6 mi (10 km). The tube face will follow the contours of the bay floor, changing direction six times vertically and twice horizontally to avoid a rock outcropping. By following the bay floor, the entire structure will lie in a bed of mud; in the event of an earthquake, the tube face would be cushioned by the mud, which is expected to absorb the shock. To permit the tube face to be displaced several inches in all directions during the motions of an earthquake, the tube face will be connected to its terminals by means of Flexible joints.
The display of a radar is either a circular cathode ray tube face (CRT) from five to twenty inches in diameter, or a rectangular "raster-scanned" view using a CRT or an LCD. The center of the face represents the position of the radar-equipped vessel and the presentation is roughly like that of a navigational chart. The presentation can be in color, or in several intensities of green (CRT) or shades of gray (LCD).
A bright radial line on the face of the tube face represents the radar beam; it rotates in synchronism with the antenna. Reflections show up as points or patches of light depending upon the size of the echo-producing object. The persistence of the screen is such that the points and patches of light do not completely fade out before the antenna has made another rotation and the lights are restored to brilliance. Thus the picture on the radarscope is repainted every few seconds. |
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