. Elementary and dental radiography / by Howard Riley Raper . edin Chapter V so much that it is difficult to comparethe two. When the current will not pass through anordinary X-ray tube, as we wish it to, we lower the vacuum of the tubeby liberating gases in it, and we may then force a variable amount ofmilliamperage through it; depending on where we place the lever of the 332 APPENDIX X-ray machine rheostat. No current at all will pass through the Coolidgetube until the tungsten filament or coil (Fig. 365) is heated, and then notmore than a certain amount will pass through, regardless of how
. Elementary and dental radiography / by Howard Riley Raper . edin Chapter V so much that it is difficult to comparethe two. When the current will not pass through anordinary X-ray tube, as we wish it to, we lower the vacuum of the tubeby liberating gases in it, and we may then force a variable amount ofmilliamperage through it; depending on where we place the lever of the 332 APPENDIX X-ray machine rheostat. No current at all will pass through the Coolidgetube until the tungsten filament or coil (Fig. 365) is heated, and then notmore than a certain amount will pass through, regardless of how far thelever of the rheostat of the X-ray machine is advanced. The degree ofheat of the tungsten filament controls the limit of the milliamperage whichmay be sent through the tube. The vacuum of the Coolidge tube is very high and remains practicallythe same always. Temperature of the tungsten filament—i. e., ionization—and not changes in degree of vacuum due to gas liberation, governsthe amount of milliamperage which may be sent through a Coolidge When a Coolidge tube is seen in operation for the fluorescence. first time the observer is surprised to find that it does not fluoresce. The heated filament is seen and, if the current is left on long enough, the target may get red or white hot, but the green fluorescence of the ordinary X-ray tube does not occur at all. If the polarity of the machine is wrong, it will be shown by the fact that the milliammeter will register no current, regardless of how high the filament temperature may be, for the Coolidge tube allows current to pass through it in only one direction; from filament to target. In the practice of radiodontia one need have littleficatina. fear of overheating a Coolidge tube. The tube will take greater quantities of current over a longer periodof time than the radiodontist will find it necessary to use, unless theCoolidge tube is one of the finest focus, when, to keep from burning thetarget, not more than abou
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